[WIP] fix(Timeline): render as unordered list

[francinelucca]

Closes https://github.com/github/primer/issues/3354

Currently the Timeline component lacks semantic structure, in that it renders like a div with divs inside of it. This doesn't convey appropriate structural meaning accessibility-wise. This PR introduces a new API under a FF to have the Timeline render as an unordered list with listitems inside of it

Introduces a new rendering behavior for Timeline under feature flag:

• if FF is off (default):
  • Timeline renders as per usual: as a div with inner divs
• if FF is on:
  • Timeline renders as per usual: as a div
  • Timeline.Item renders as an li
  • It is expected that all Timeline.Item will be wrapped inside a Timeline.Group (new component) that renders as an ul

Changelog

New

• primer_react_timeline_as_list feature flag (defaults to false)
• New Timeline.Group subcomponent (used to wrap Timeline.Items within a list when FF is on
• Add new Timeline.Group subcomponent to Timeline *.docs.json
• Tests for new Timeline behaviors

Changed

• Turn on new primer_react_timeline_as_list FF in SB Timeline stories, use new API
• Added css styles to Timeline to avoid visual changes when switching to ul
• Render TimelineItem as li if primer_react_timeline_as_list FF is on (div otherwise)
• Add aria-hidden to Timeline.Break if primer_react_timeline_as_list FF is on
• Update Timeline snapshot

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

1- Release new API under FF (this PR)
2 - Move dotcom to new API (upcoming separate PR, after this is merged)
3 - Deprecate "old" API
4 - Remove old API in next major

Testing & Reviewing

Revise deployment stories, see DOM renders as list and no visual regressions

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: 3f15458

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 depends on. If you are GitHub staff, we recommend testing these changes with github/github using the integration workflow. Thanks!

[github-actions]

size-limit report 📦

Path	Size
packages/react/dist/browser.esm.js	98.73 KB (+0.15% 🔺)
packages/react/dist/browser.umd.js	99 KB (+0.07% 🔺)

[primer-integration]

👋 Hi from github/github! Your integration PR is ready: https://github.com/github/github/pull/350084

[primer-integration]

🟢 golden-jobs completed with status success.