## Objective

Continuation of #16999.

This PR handles the following:
- Many hierarchy-related commands are wrappers around `World` and
`EntityWorldMut` methods and can be moved to closures:
  - `AddChild`
  - `InsertChildren`
  - `AddChildren`
  - `RemoveChildren`
  - `ClearChildren`
  - `ReplaceChildren`
  - `RemoveParent`
  - `DespawnRecursive`
  - `DespawnChildrenRecursive`
  - `AddChildInPlace`
  - `RemoveParentInPlace`
- `SendEvent` is a wrapper around `World` methods and can be moved to a
closure (and its file deleted).

## Migration Guide

If you were queuing the structs of hierarchy-related commands or
`SendEvent` directly, you will need to change them to the methods
implemented on `EntityCommands` (or `Commands` for `SendEvent`):

| Struct | Method |

|--------------------------------------------------------------------|---------------------------------------------------------------------------------------------|
| `commands.queue(AddChild { child, parent });` |
`commands.entity(parent).add_child(child);` OR
`commands.entity(child).set_parent(parent);` |
| `commands.queue(AddChildren { children, parent });` |
`commands.entity(parent).add_children(children);` |
| `commands.queue(InsertChildren { children, parent });` |
`commands.entity(parent).insert_children(children);` |
| `commands.queue(RemoveChildren { children, parent });` |
`commands.entity(parent).remove_children(children);` |
| `commands.queue(ReplaceChildren { children, parent });` |
`commands.entity(parent).replace_children(children);` |
| `commands.queue(ClearChildren { parent });` |
`commands.entity(parent).clear_children();` |
| `commands.queue(RemoveParent { child });` |
`commands.entity(child).remove_parent()` |
| `commands.queue(DespawnRecursive { entity, warn: true });` |
`commands.entity(entity).despawn_recursive();` |
| `commands.queue(DespawnRecursive { entity, warn: false });` |
`commands.entity(entity).try_despawn_recursive();` |
| `commands.queue(DespawnChildrenRecursive { entity, warn: true });` |
`commands.entity(entity).despawn_descendants();` |
| `commands.queue(DespawnChildrenRecursive { entity, warn: false});` |
`commands.entity(entity).try_despawn_descendants();` |
| `commands.queue(SendEvent { event });` | `commands.send_event(event);`
|

…tyWorldMut` (bevyengine#16999)

## Objective

Commands were previously limited to structs that implemented `Command`.
Now there are blanket implementations for closures, which (in my
opinion) are generally preferable.

Internal commands within `commands/mod.rs` have been switched from
structs to closures, but there are a number of internal commands in
other areas of the engine that still use structs. I'd like to tidy these
up by moving their implementations to methods on
`World`/`EntityWorldMut` and changing `Commands` to use those methods
through closures.

This PR handles the following:
- `TriggerEvent` and `EmitDynamicTrigger` double as commands and helper
structs, and can just be moved to `World` methods.
- Four structs that enabled insertion/removal of components via
reflection. This functionality shouldn't be exclusive to commands, and
can be added to `EntityWorldMut`.
- Five structs that mostly just wrapped `World` methods, and can be
replaced with closures that do the same thing.

## Solution

- __Observer Triggers__ (`observer/trigger_event.rs` and
`observer/mod.rs`)
- Moved the internals of `TriggerEvent` to the `World` methods that used
it.
  - Replaced `EmitDynamicTrigger` with two `World` methods:
    - `trigger_targets_dynamic`
    - `trigger_targets_dynamic_ref`
- `TriggerTargets` was now the only thing in
`observer/trigger_event.rs`, so it's been moved to `observer/mod.rs` and
`trigger_event.rs` was deleted.
- __Reflection Insert/Remove__ (`reflect/entity_commands.rs`)
- Replaced the following `Command` impls with equivalent methods on
`EntityWorldMut`:
    - `InsertReflect` -> `insert_reflect`
    - `InsertReflectWithRegistry` -> `insert_reflect_with_registry`
    - `RemoveReflect` -> `remove_reflect`
    - `RemoveReflectWithRegistry` -> `remove_reflect_with_registry`
- __System Registration__ (`system/system_registry.rs`)
- The following `Command` impls just wrapped a `World` method and have
been replaced with closures:
    - `RunSystemWith`
    - `UnregisterSystem`
    - `RunSystemCachedWith`
    - `UnregisterSystemCached`
- `RegisterSystem` called a helper function that basically worked as a
constructor for `RegisteredSystem` and made sure it came with a marker
component. That helper function has been replaced with
`RegisteredSystem::new` and a `#[require]`.

## Possible Addition

The extension trait that adds the reflection commands,
`ReflectCommandExt`, isn't strictly necessary; we could just `impl
EntityCommands`. We could even move them to the same files as the main
impls and put it behind a `#[cfg]`.

The PR that added it [had a similar
conversation](bevyengine#8895 (comment))
and decided to stick with the trait, but we could revisit it here if so
desired.

## Objective

Continuation of bevyengine#16999.

This PR handles the following:
- Many hierarchy-related commands are wrappers around `World` and
`EntityWorldMut` methods and can be moved to closures:
  - `AddChild`
  - `InsertChildren`
  - `AddChildren`
  - `RemoveChildren`
  - `ClearChildren`
  - `ReplaceChildren`
  - `RemoveParent`
  - `DespawnRecursive`
  - `DespawnChildrenRecursive`
  - `AddChildInPlace`
  - `RemoveParentInPlace`
- `SendEvent` is a wrapper around `World` methods and can be moved to a
closure (and its file deleted).

## Migration Guide

If you were queuing the structs of hierarchy-related commands or
`SendEvent` directly, you will need to change them to the methods
implemented on `EntityCommands` (or `Commands` for `SendEvent`):

| Struct | Method |

|--------------------------------------------------------------------|---------------------------------------------------------------------------------------------|
| `commands.queue(AddChild { child, parent });` |
`commands.entity(parent).add_child(child);` OR
`commands.entity(child).set_parent(parent);` |
| `commands.queue(AddChildren { children, parent });` |
`commands.entity(parent).add_children(children);` |
| `commands.queue(InsertChildren { children, parent });` |
`commands.entity(parent).insert_children(children);` |
| `commands.queue(RemoveChildren { children, parent });` |
`commands.entity(parent).remove_children(children);` |
| `commands.queue(ReplaceChildren { children, parent });` |
`commands.entity(parent).replace_children(children);` |
| `commands.queue(ClearChildren { parent });` |
`commands.entity(parent).clear_children();` |
| `commands.queue(RemoveParent { child });` |
`commands.entity(child).remove_parent()` |
| `commands.queue(DespawnRecursive { entity, warn: true });` |
`commands.entity(entity).despawn_recursive();` |
| `commands.queue(DespawnRecursive { entity, warn: false });` |
`commands.entity(entity).try_despawn_recursive();` |
| `commands.queue(DespawnChildrenRecursive { entity, warn: true });` |
`commands.entity(entity).despawn_descendants();` |
| `commands.queue(DespawnChildrenRecursive { entity, warn: false});` |
`commands.entity(entity).try_despawn_descendants();` |
| `commands.queue(SendEvent { event });` | `commands.send_event(event);`
|

…tyWorldMut` (bevyengine#16999)

## Objective

Commands were previously limited to structs that implemented `Command`.
Now there are blanket implementations for closures, which (in my
opinion) are generally preferable.

Internal commands within `commands/mod.rs` have been switched from
structs to closures, but there are a number of internal commands in
other areas of the engine that still use structs. I'd like to tidy these
up by moving their implementations to methods on
`World`/`EntityWorldMut` and changing `Commands` to use those methods
through closures.

This PR handles the following:
- `TriggerEvent` and `EmitDynamicTrigger` double as commands and helper
structs, and can just be moved to `World` methods.
- Four structs that enabled insertion/removal of components via
reflection. This functionality shouldn't be exclusive to commands, and
can be added to `EntityWorldMut`.
- Five structs that mostly just wrapped `World` methods, and can be
replaced with closures that do the same thing.

## Solution

- __Observer Triggers__ (`observer/trigger_event.rs` and
`observer/mod.rs`)
- Moved the internals of `TriggerEvent` to the `World` methods that used
it.
  - Replaced `EmitDynamicTrigger` with two `World` methods:
    - `trigger_targets_dynamic`
    - `trigger_targets_dynamic_ref`
- `TriggerTargets` was now the only thing in
`observer/trigger_event.rs`, so it's been moved to `observer/mod.rs` and
`trigger_event.rs` was deleted.
- __Reflection Insert/Remove__ (`reflect/entity_commands.rs`)
- Replaced the following `Command` impls with equivalent methods on
`EntityWorldMut`:
    - `InsertReflect` -> `insert_reflect`
    - `InsertReflectWithRegistry` -> `insert_reflect_with_registry`
    - `RemoveReflect` -> `remove_reflect`
    - `RemoveReflectWithRegistry` -> `remove_reflect_with_registry`
- __System Registration__ (`system/system_registry.rs`)
- The following `Command` impls just wrapped a `World` method and have
been replaced with closures:
    - `RunSystemWith`
    - `UnregisterSystem`
    - `RunSystemCachedWith`
    - `UnregisterSystemCached`
- `RegisterSystem` called a helper function that basically worked as a
constructor for `RegisteredSystem` and made sure it came with a marker
component. That helper function has been replaced with
`RegisteredSystem::new` and a `#[require]`.

## Possible Addition

The extension trait that adds the reflection commands,
`ReflectCommandExt`, isn't strictly necessary; we could just `impl
EntityCommands`. We could even move them to the same files as the main
impls and put it behind a `#[cfg]`.

The PR that added it [had a similar
conversation](bevyengine#8895 (comment))
and decided to stick with the trait, but we could revisit it here if so
desired.

## Objective

Continuation of bevyengine#16999.

This PR handles the following:
- Many hierarchy-related commands are wrappers around `World` and
`EntityWorldMut` methods and can be moved to closures:
  - `AddChild`
  - `InsertChildren`
  - `AddChildren`
  - `RemoveChildren`
  - `ClearChildren`
  - `ReplaceChildren`
  - `RemoveParent`
  - `DespawnRecursive`
  - `DespawnChildrenRecursive`
  - `AddChildInPlace`
  - `RemoveParentInPlace`
- `SendEvent` is a wrapper around `World` methods and can be moved to a
closure (and its file deleted).

## Migration Guide

If you were queuing the structs of hierarchy-related commands or
`SendEvent` directly, you will need to change them to the methods
implemented on `EntityCommands` (or `Commands` for `SendEvent`):

| Struct | Method |

|--------------------------------------------------------------------|---------------------------------------------------------------------------------------------|
| `commands.queue(AddChild { child, parent });` |
`commands.entity(parent).add_child(child);` OR
`commands.entity(child).set_parent(parent);` |
| `commands.queue(AddChildren { children, parent });` |
`commands.entity(parent).add_children(children);` |
| `commands.queue(InsertChildren { children, parent });` |
`commands.entity(parent).insert_children(children);` |
| `commands.queue(RemoveChildren { children, parent });` |
`commands.entity(parent).remove_children(children);` |
| `commands.queue(ReplaceChildren { children, parent });` |
`commands.entity(parent).replace_children(children);` |
| `commands.queue(ClearChildren { parent });` |
`commands.entity(parent).clear_children();` |
| `commands.queue(RemoveParent { child });` |
`commands.entity(child).remove_parent()` |
| `commands.queue(DespawnRecursive { entity, warn: true });` |
`commands.entity(entity).despawn_recursive();` |
| `commands.queue(DespawnRecursive { entity, warn: false });` |
`commands.entity(entity).try_despawn_recursive();` |
| `commands.queue(DespawnChildrenRecursive { entity, warn: true });` |
`commands.entity(entity).despawn_descendants();` |
| `commands.queue(DespawnChildrenRecursive { entity, warn: false});` |
`commands.entity(entity).try_despawn_descendants();` |
| `commands.queue(SendEvent { event });` | `commands.send_event(event);`
|