[material_ui] Standardize M3 and M3E token template base class names

[QuncCccccc]

As title. Related to flutter/flutter#187899

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[QuncCccccc]

The generated file is still follow <component_name>_defaults_m3.g.dart, maybe we update it when we migrate?

[gemini-code-assist]

Code Review

This pull request renames several template classes and subclasses across the gen_defaults tool, tests, and documentation to standardize naming conventions (e.g., renaming M3TokenTemplate to TokenTemplateM3). Feedback recommends restoring a blank line before a test block in gen_defaults_test.dart to maintain consistent spacing and readability.

[elliette]

The generated file is still follow <component_name>_defaults_m3.g.dart, maybe we update it when we migrate?

Yes, sounds good! I think right now they don't have the _m3 suffix, so we can add it when we move the files.

[elliette]

Thank you for opening a separate PR for this, LGTM!