[🔨 WIP] Support responsive overflow in ButtonGroup

[hectahertz]

Closes https://github.com/github/primer/issues/6096

NOTE: ⚠️ This is a WIP PR, the approach here can be changed as there's multiple ways of tackling this issue.

• Added automatic overflow detection and handling in ButtonGroup.tsx, including logic to measure button widths, calculate how many buttons fit, and move excess buttons into an overflow menu triggered by an icon button. The overflow menu uses AnchoredOverlay for accessibility and keyboard navigation.
• Introduced the overflowAriaLabel prop to ButtonGroup, with a default value of "More actions", ensuring the overflow trigger button is accessible to screen readers.

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• [ ] 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)

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Latest commit: 1f8a13350a7e1cc2af7e8ca78e7f087945669804

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[github-actions[bot]]

:wave: 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.