[material_ui] Add contrastLevel for M3 ColorScheme

[QuncCccccc]

Fixes flutter/flutter#184951

This PR is to add medium- and high-contrast M3 ColorScheme baselines. I also added a double? contrastLevel to ThemeData.

By default, ThemeData() shows standard M3 ColorScheme. To choose medium/high contrast, use

ThemeData(
  // brightness: Brightness.dark shows dark version of the baselines.
  contrastLevel: ContrastLevel.medium, // ContrastLevel.high
)

This change will not cause breaking changes because the standard-contrast m3 color scheme baseline stays the same.

TODO:
Add an example or update an existing example to reflect the new feature.

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• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
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• 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 ///).
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• All existing and new tests are passing.

[QuncCccccc]

We don't need a class for ColorScheme, so I added a requiresGeneratedClass field.

[gemini-code-assist]

Code Review

This pull request introduces support for Material 3 contrast levels in ThemeData by adding a contrastLevel parameter to the constructor. This allows standard, medium, high, or custom contrast levels to be applied when generating or selecting a ColorScheme. The generation of default color schemes is migrated to a new template (ColorSchemeTemplateM3), and corresponding unit tests are added. Feedback on the changes points out a broken documentation link referencing a non-existent public ThemeData.contrastLevel property, suggesting it be updated to refer to the constructor parameter instead.

[gemini-code-assist]

The documentation references [ThemeData.contrastLevel], but ThemeData does not actually have a public contrastLevel property or getter (similar to colorSchemeSeed). This results in a broken documentation link. We should update the documentation to refer to the constructor parameter instead.

/// A custom double between -1.0 and 1.0 can also be passed as the
/// contrastLevel parameter of the [ThemeData] constructor when
/// [ThemeData.colorSchemeSeed] is provided, in which case the value is passed
/// to [ColorScheme.fromSeed].

[QuncCccccc]

Updated documentation.