gen_defaults should follow new versioning strategy

[elliette]

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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[gemini-code-assist]

Code Review

This pull request refactors the token template generation tool to support both Material 3 and Material 3 Expressive defaults. It introduces a public TokenTemplate class, dynamically generates class names based on the Material version, and adds assertions to ensure the generated contents define the expected class. The tests have been updated to cover both template types. Feedback suggests handling underscores in template names when generating class names to ensure they strictly adhere to the Dart UpperCamelCase style guide.

[gemini-code-assist]

Code Review

This pull request refactors the TokenTemplate class to standardize template naming conventions using spaced, capitalized words, and automatically derives output filenames and class names for Material 3 and Material 3 Expressive defaults. It also adds assertions to validate the generated class names and template name formats, and updates the corresponding tests and test fixtures. Feedback on the pull request points out that using parentName in the part of directive will generate an invalid filename containing spaces and capitalization, and suggests using $snakeName.dart instead.

[QuncCccccc]

LGTM! Thanks!