feat(BaseStyles): Convert BaseStyles to CSS modules behind team feature flag

[JelloBagel]

Closes https://github.com/github/primer/issues/4358

Changelog

Changed

• Convert BaseStyles to CSS modules behind primer_react_css_modules_team feature flag

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Testing & Reviewing

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github (Learn more about how to run integration tests)

[changeset-bot]

🦋 Changeset detected

Latest commit: d91d756

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/react	Minor

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[github-actions]

👋 Hi, this pull request contains changes to the source code that github/github depends on. If you are GitHub staff, we recommend testing these changes with github/github using the integration workflow. Thanks!

[github-actions]

size-limit report 📦

Path	Size
packages/react/dist/browser.esm.js	102.28 KB (-0.11% 🔽)
packages/react/dist/browser.umd.js	102.64 KB (-0.03% 🔽)

[primer-integration]

👋 Hi from github/github! Your integration PR is ready: https://github.com/github/github/pull/353826

[JelloBagel]

The dotcom integration test failures seem unrelated to this PR, citing similar test failure from a slack thread: https://github.slack.com/archives/C07QUP8G909/p1733413713531239

[primer-integration]

🟢 golden-jobs completed with status success.

[jonrohan]

Tested in dotcom and everything seems good.

[github-actions]

👋 Hi, there are new commits since the last successful integration test. We recommend running the integration workflow once more, unless you are sure the new changes do not affect github/github. Thanks!

[github-actions]

👋 Hi, there are new commits since the last successful integration test. We recommend running the integration workflow once more, unless you are sure the new changes do not affect github/github. Thanks!