migrate TreeView to CSS modules

[ktravers]

closes https://github.com/github/primer/issues/4407

• Helps with https://github.com/github/pull-requests/issues/11167
• Related: https://github.com/github/primer/issues/3159

This PR migrates the TreeView component from using styled system props to CSS modules. It follows the instructions listed in the Primer React Styled Components to CSS Modules Refactoring Guide:

• Verify VRT (Visual Regression Testing) Coverage (see existing e2e/components/TreeView.test.ts)
• Ensure All Visual Changes Are Completed (confirmed by checking open issues related to TreeView)
• Ensure the component accepts a className on the top DOM level only for styling from outside of primer/react
• Ensure the component accepts a style prop for more dynamic styling like positioning
• Add a feature flag for controlled rollout (staff shipping)
• Ensure Component still accepts sx styling

Before	After

Changelog

New

• Adds support for style prop to TreeView component

Changed

• Updates TreeView component to support CSS Modules (behind feature flag)
• Fixes minor test and documentation typos

Removed

N/A

Rollout strategy

• Patch release
• Minor release. See https://github.com/github/primer/discussions/3911
• Major release; if selected, include a written rollout or migration plan.
• None; if selected, include a brief description as to why

Testing & Reviewing

These changes are easier to review with whitespace changes hidden.

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook) (N/A)
• 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: 919f426

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	99.12 KB (+0.44% 🔺)
packages/react/dist/browser.umd.js	99.46 KB (+0.35% 🔺)

[ktravers]

Copied styles directly from TreeView component, then migrated to CSS variables following this guide: https://primer.style/foundations/primitives/migrating#migrating-from-sx-props-primer-react

[ktravers]

Taking back to draft while I look into new VRT failures...

[primer-integration]

👋 Hi from github/github! Your integration PR is ready: https://github.com/github/github/pull/350953

[primer-integration]

🔴 golden-jobs completed with status failure.

[langermank]

Approving the VRT updates! Before merging, we should confirm that the dotcom integration test failure isn't related (@jonrohan can take a look)

[jonrohan]

Yep these look like the same integration errors, not related to this pr