[material_ui] Add Material 3 Expressive IconButton

[QuncCccccc]

This PR is to port #12093 which was created in m3e_migration feature branch.

The original PR description

This PR is to add Material 3 Expressive IconButton support behind an explicit component-level opt-in.

This PR adds:

• 3 new ButtonStyle properties used by the IconButton M3E migration:
  • sizeVariant
  • iconButtonWidth
  • shapeVariant
• IconButtonThemeData.variant so IconButtons can opt into StyleVariant.material3Expressive.
• IconButtonTemplateM3E to generate IconButton defaults from the M3E token data.
• Adds the generated M3E IconButton defaults in:
  • packages/material_ui/lib/src/generated/icon_button_defaults_m3e.g.dart

TODO:

• I'll add a separate PR/demo showing the Material 3 Expressive IconButton variants.
• Will also update IconButton shape morphing to use physics-based motion animation.

Pre-Review Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [AI contribution guidelines] and understand my responsibilities, or I am not using AI tools.
• I read the [Tree Hygiene] page, which explains my responsibilities.
• I read and followed the [relevant style guides] and ran [the auto-formatter].
• I signed the [CLA].
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I [linked to at least one issue that this PR fixes] in the description above.
• 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].
• 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].
• All existing and new tests are passing.

[gemini-code-assist]

Code Review

This pull request introduces Material 3 Expressive support for the IconButton component. It adds new size, width, and shape variants, updates the ButtonStyle class to include these new properties, and implements the necessary defaults in a generated file. A code review comment correctly identified that the generated code uses the deprecated MaterialStatePropertyAll instead of the preferred WidgetStatePropertyAll, which should be updated to ensure modern API usage.

[gemini-code-assist]

In modern Flutter, MaterialStatePropertyAll is deprecated in favor of WidgetStatePropertyAll. Since this is a new template generating new code, we should use WidgetStatePropertyAll to avoid deprecation warnings.

Please also apply this replacement to other template methods in this file, such as _generateStandardDefaults, _generateFilledDefaults, _generateFilledTonalDefaults, and _generateOutlinedDefaults.

  String get _sizeDependentProperties {
    return '''
  @override
  WidgetStateProperty<EdgeInsetsGeometry>? get padding =>
    WidgetStatePropertyAll<EdgeInsetsGeometry>($_paddingSwitch);

  @override
  WidgetStateProperty<Size>? get minimumSize =>
    WidgetStatePropertyAll<Size>($_minimumSizeSwitch);

  @override
  WidgetStateProperty<Size>? get maximumSize =>
    const WidgetStatePropertyAll<Size>(Size.infinite);

  @override
  WidgetStateProperty<double>? get iconSize =>
    WidgetStatePropertyAll<double>($_iconSizeSwitch);

  @override
  WidgetStateProperty<OutlinedBorder>? get shape =>
    WidgetStateProperty.resolveWith((Set<WidgetState> states) {
      if (states.contains(WidgetState.pressed)) {
        return $_pressedShapeSwitch;
      }
      if (toggleable && states.contains(WidgetState.selected)) {
        return $_selectedShapeSwitch;
      }
      return $_containerShapeSwitch;
    });
''';
  }