WIP: Fix UnderlineNav CLS by using wrapping to overflow items onto the next (hidden) line

[iansan5653]

This is just a quick proof of concept to show a CSS-based approach to hiding overflowing items. It uses flex-wrap to wrap them onto the next line, along with overflow: hidden and an explicit height to clip subsequent lines.

In addition, this shows how we can use CSS scroll-state container queries to determine whether or not to show the tab icons and overflow menu initially.

This is a minimal and quick PR - after the initial render, all the logic is still done by JS-based width calculations.

For more context, see https://github.com/github/primer/issues/6291#issuecomment-3861950186

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

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[github-actions]

👋 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.