[material_ui] Add helper methods in gen_defaults template

[QuncCccccc]

Related to flutter/flutter#184950

This PR is to add some helper methods to handle color, and shape tokens. Also I updated the generated default class name from _M3xxx to _xxxM3 to keep the name consistent with the original M2 and M3 defaults.

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[gemini-code-assist]

Code Review

This pull request updates the Material 3 component defaults generator by renaming the generated class suffixes and adding helper methods to generate Dart expressions for token numbers, colors, and shapes. It also updates the corresponding tests and test fixtures to cover these new capabilities. Review feedback suggests optimizing the colorWithOpacity helper to avoid redundant calls when opacity is 1.0, and correcting a minor formatting issue by adding a space after the comma in the generated BorderRadius.vertical expression and its corresponding test.

[elliette]

LGTM!