part of flutter/flutter#184950

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Co-authored-by: Flutter Material Tokens Copybara Bot <[email protected]>

Work towards flutter/flutter#186906

Adds the initial `gen_defaults` sub-directory with the initial
`TokenTemplate` and generator. This is simply the initial scaffold, we
will need to add additional helper functions as necessary as we begin
adding the actual templates.

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Work towards flutter/flutter#186906
Work towards flutter/flutter#187899

Previously, the `gen_defaults` script generated the files in different
directories based on whether or not the template was an M3 or M3E
template. Since we are planning on using an variant enum instead of
different libraries to separate M3 and M3E implementations, the
`gen_defaults` script needs to be accordingly updated.

**Before:**

```
lib/src/
├── button.dart                      ← Editable M3 parent file
├── material_3_expressive/
│   └── button.dart                  ← Editable M3E parent file
└── generated/
    ├── button_defaults.g.dart       ← Generated M3 part file
    └── material_3_expressive/
        └── button_defaults.g.dart   ← Generated M3E part file
```

**After:**

```
lib/src/
├── button.dart                              ← Editable M3/M3E parent file
└── generated/
    ├── button_m3_defaults.g.dart            ← Generated M3 part file
    └── button_m3e_defaults.g.dart           ← Generated M3E part file
```


The template now creates the generated class name as well, either
`M3NameDefaults` or `M3ENameDefaults` (e.g. `M3EButtonDefaults`).


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…_excluded/` before migration (#12206)

…12209)

As title. Related to flutter/flutter#187899


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…pt (#12122)

~Will land #12209 first.~

Fixes flutter/flutter#188397

Initially gen_defaults script and its templates were used to handle JSON
tokens. After we fix the token pipeline and make it possible to get M3E
tokens, we use Dart constants and re-created the gen_defaults in
material_ui. After decoupling, the old gen_defaults will be deprecated
soon.

This PR is to:
* create a template for M3 `AppBar` so we can use new script to generate
M3 defaults.
* move the generated code to `generated/` folder so all M3 and M3E
generated code will be in the same location.

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…ts` templates (#12220)

Related to https://github.com/orgs/flutter/projects/250

This PR is just to add the `StyleVariant` enum so we can start migrating
individual components.

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…lts` (#12223)

Adds `name` field to `TypescaleStruct` to ensure Flutter can access the
original typescale name (e.g., 'body-medium' or
'variable.emphasized.title-small') directly from the struct.

Implementation changes in cl/960411657

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Co-authored-by: Flutter Material Tokens Copybara Bot <[email protected]>

#12219)

Work towards flutter/flutter#187899
Fixes flutter/flutter#188396

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[semantic versioning]:
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[repository CHANGELOG style]:
https://github.com/flutter/flutter/blob/master/docs/ecosystem/contributing/README.md#changelog-style
[test exemption]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Tree-hygiene.md#tests*Replace
this paragraph with a description of what this PR is changing or adding,
and why. Consider including before/after screenshots.*

…12218)

Work towards flutter/flutter#187899
Fixes flutter/flutter#188401

## Pre-Review Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [AI contribution guidelines] and understand my
responsibilities, or I am not using AI tools.
- [x] I read the [Tree Hygiene] page, which explains my
responsibilities.
- [x] I read and followed the [relevant style guides] and ran [the
auto-formatter].
- [x] I signed the [CLA].
- [x] The title of the PR starts with the name of the package surrounded
by square brackets, e.g. `[shared_preferences]`
- [x] I [linked to at least one issue that this PR fixes] in the
description above.
- [x] I followed [the version and CHANGELOG instructions], using
[semantic versioning] and the [repository CHANGELOG style], or I have
commented below to indicate which documented exception this PR falls
under[^1].
- [x] I updated/added any relevant documentation (doc comments with
`///`).
- [ ] I added new tests to check the change I am making, or I have
commented below to indicate which [test exemption] this PR falls
under[^1].
- [x] All existing and new tests are passing.

…12227)

Work towards flutter/flutter#187899
Fixes flutter/flutter#188402

## Pre-Review Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [AI contribution guidelines] and understand my
responsibilities, or I am not using AI tools.
- [x] I read the [Tree Hygiene] page, which explains my
responsibilities.
- [x] I read and followed the [relevant style guides] and ran [the
auto-formatter].
- [x] I signed the [CLA].
- [x] The title of the PR starts with the name of the package surrounded
by square brackets, e.g. `[shared_preferences]`
- [x] I [linked to at least one issue that this PR fixes] in the
description above.
- [x] I followed [the version and CHANGELOG instructions], using
[semantic versioning] and the [repository CHANGELOG style], or I have
commented below to indicate which documented exception this PR falls
under[^1].
- [x] I updated/added any relevant documentation (doc comments with
`///`).
- [ ] I added new tests to check the change I am making, or I have
commented below to indicate which [test exemption] this PR falls
under[^1].
- [x] All existing and new tests are passing.