Add Material style variant enum

[QuncCccccc]

Related to Bring Material 3 Expressive to Flutter

This PR is just to add the StyleVariant enum so we can start migrating individual components.

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.

[QuncCccccc]

Seems no need to add tests. I'll request test exemption when we merge changes to main (after v1.0.0).

[gemini-code-assist]

Code Review

This pull request introduces the StyleVariant enum to theme_data.dart, defining the material3 and material3Expressive style variants. The review feedback suggests renaming StyleVariant to MaterialStyleVariant to prevent namespace pollution and maintain consistency with other Material-specific enums, alongside a minor capitalization correction in the documentation.

[dkwingsmt]

LGTM

[dkwingsmt]

Should we hold this PR to after the v1.0.0 release? I thought the v1.0.0 release was meant to be as close to flutter/material as possible.

[QuncCccccc]

Should we hold this PR to after the v1.0.0 release?

Yes, this PR will only be merged in m3e_migration feature branch. Once we have v1.0.0 release, we will move the commits in this feature branch to main:). So we will not have any expressive-related change before v1.0.0 in material_ui.

[dkwingsmt]

That's awesome! Didn't notice it's targeting a new branch.