Add components to support skeleton loading states

[mperrotti]

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

Kapture.2023-12-11.at.17.02.20.mp4

Changelog

New

Adds the following components:

• SkeletonBox - 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
• 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)

[changeset-bot]

🦋 Changeset detected

Latest commit: 2f8d800

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]

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% 🔺)

[mperrotti]

It's possible that somebody could render a skeleton loader on top of something with the bgColor-muted background color. This would make the loader invisible except for the "shimmer" animation.

Reviewers, how do you think we should reckon with this?

1. Add a boolean prop to change the color of a skeleton loaders. Kind of like the contrast prop on TextInput.
2. Add a component-specific color token that uses a color from the base gray scale with higher contrast.
3. Keep this as-is until we identify a real use-case where this causes a problem.