Add React-first version of UnderlinePanels

[adierkens]

Introduces a new UnderlinePanels2 component which utilizes the new Tabs utility. This is intended to address the usability issues that have come up with the mixing of tab-container-element backing the current UnderlinePanels component. See #7093, #7092, #6253, #6250

I explicitly copied over the VRT snapshots from the current component to ensure 1:1 consistency with the behavior. All stories have also been copied to mirror the current component.

The goal is to convert usage of the current UnderlinePanels component to the new one for removal in a future major version.

Changelog

New

New UnderlinePanels2 component as a replacement for the existing UnderlinePanels

Rollout strategy

• Minor release

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: 19bdfe3

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-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-integration]

😢 Hi from github/github-ui. The integration workflow has failed: https://github.com/github/github-ui/actions/runs/19583716011