What does this implement/fix? Explain your changes.

This PR refactors the Model class to reduce code complexity and improve maintainability and DX. More specifically:

• Model::$data is now typed via a Generic type, instead of needlessly type-hinting in child classes.
  As a result, many doc types have been updated to use that generic instead of mixed or a lower-fidelity type.

• Splits the wrap_fields() method into several smaller functions (Single Responsibilities principle)

• Fixes the graphql_model_field_capability filter, so it runs on all fields, not just those that are registered with a capability property.
  This allows the filter to be used to add capability checks to existing fields, and not just to modify preset capabilities.

• Clarifies usage of various functions, specifically that setup() and tear_down() functions apply per field.

Files that rely on the Model class have been cleaned up accordingly.

Developers Note

To correctly type the TData generic used by the Model, you can use the @extends syntax on the class doc-block:

For example if you are modeling data sourced from a custom WP_Post object:

{...}

use \WPGraphQL\Model\Model;

/**
 * {other class-docblock meta}
 *
 * @extends \WPGraphQL\Model\Model<\WP_Post>
 */
class EventCptModel extends Model { ... }

Does this close any currently open issues?

Any other comments?

• AbstractDataLoader is using a @phpstan-type to centrally suppress the generic warning. In the future we can just swap that out with a @template or @extends without disrupting the rest of the file again.