AnchoredOverlay: Add support for CSS anchor positioning

[TylerJDev]

Closes https://github.com/github/primer/issues/6446

• Added support for CSS Anchor Positioning in AnchoredOverlay, gated by the primer_react_css_anchor_positioning feature flag.
• Updated ActionMenu to ensure that the anchor button receives the correct anchor class for CSS anchor positioning, and improved merging of class names between anchor and button props.
• Added new stories to AnchoredOverlay.features.stories.tsx to demonstrate anchor positioning in various layouts, including centered overlays, grid layouts, within dialogs (including nested and overflowing dialogs), scroll containers, and sticky headers.

Changelog

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• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
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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-ui (Learn more about how to run integration tests)

[changeset-bot]

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[github-actions]

⚠️ Action required

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. Or, apply the integration-tests: skipped manually label to skip these checks.

[primer]

🤖 Lint and formatting issues have been automatically fixed and committed to this PR.

[primer]

🤖 Lint issues have been automatically fixed and committed to this PR.

[primer]

🤖 Lint issues have been automatically fixed and committed to this PR.

[primer]

🤖 Lint and formatting issues have been automatically fixed and committed to this PR.

[primer]

🤖 Lint issues have been automatically fixed and committed to this PR.

[primer]

🤖 Lint issues have been automatically fixed and committed to this PR.

[siddharthkp]

🤖 Lint and formatting issues have been automatically fixed and committed to this PR.

it keeps saying that but where is the commit? 😅

[TylerJDev]

🤖 Lint and formatting issues have been automatically fixed and committed to this PR.

it keeps saying that but where is the commit? 😅

The bot really doesn't like my PR 😆

[primer-integration]

👋 Hi from github/github-ui! Your integration PR is ready: https://github.com/github/github-ui/pull/14804

[primer-integration]

Integration test results from github/github-ui:

  CI Passed
  VRT Passed
  Projects Passed

All checks passed!