Add components to support skeleton loading states

[mperrotti]

Relates to https://github.com/github/primer/issues/2682

https://github.com/primer/react/assets/2313998/854b82e1-8c52-48e1-a2ab-a95911df12cf

Changelog

New

Adds the following components:

• SkeletonBone - general-use skeleton loading component
• SkeletonAvatar - used to replace avatars with a skeleton loading state
• SkeletonText - used to replace text with a skeleton loading state

Changed

DataTable and TreeView use the new skeleton loading components instead of their ad-hoc skeleton loaders

Removed

Rollout strategy

• [ ] Patch release
• [x] 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

• [x] Added/updated tests
• [x] Added/updated documentation
• [x] Added/updated previews (Storybook)
• [x] Changes are SSR compatible
• [x] Tested in Chrome
• [x] 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[bot]]

🦋 Changeset detected

Latest commit: 2f8d800b77510ae092582cd890c18c2afd9e66c4

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[bot]]

size-limit report 📦

Path	Size
packages/react/dist/browser.esm.js	90.32 KB (+0.67% 🔺)
packages/react/dist/browser.umd.js	90.6 KB (+0.76% 🔺)

[joshblack]

@mperrotti just a heads up, started the API PR over in: https://github.com/github/primer/pull/2958 based on our conversation before break! Let me know if I missed anything in the API block and feel free to add to it. My hope is to add some usage examples before marking it as ready for review 👍

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[mperrotti]

@joshblack - I was able to address almost all of your feedback.

The only thing I struggled with was replacing sx with className on SkeletonBox.