Add x variant to unordered lists, and leadingVisual, variant and color props to list items

[rezrah]

Summary

Resolves #527

Adds granular control over list appearance.

🔗 Storybook examples

List of notable changes:

• added variant="x" prop to UnorderedList component.
• added variant prop to ListItem component, to allow alternating between default and muted colors
• added leadingVisual prop to ListItem to allow custom icon overrides
• added leadingVisualFill prop to ListItem to allow custom icon colors
• added leadingVisualAriaLabel prop to ListItem to allow aria labels to be forwarded to the icon

Steps to test:

1. Open and review the following storybook stories
2. Review the updated documentation

Contributor checklist:

• All new and existing CI checks pass
• Tests prove that the feature works and covers both happy and unhappy paths
• Any drop in coverage, breaking changes or regressions have been documented above
• New visual snapshots have been generated / updated for any UI changes
• All developer debugging and non-functional logging has been removed
• Related issues have been referenced in the PR description

Reviewer checklist:

• Check that pull request and proposed changes adhere to our contribution guidelines and code of conduct
• Check that tests prove the feature works and covers both happy and unhappy paths
• Check that there aren't other open Pull Requests for the same update/change

Screenshots:

[changeset-bot]

🦋 Changeset detected

Latest commit: dd85369

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 6 packages

Name	Type
@primer/react-brand	Patch
@primer/brand-primitives	Patch
@primer/brand-e2e	Patch
@primer/brand-fonts	Patch
@primer/brand-config	Patch
@primer/brand-storybook	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[github-actions]

🟢 No design token changes found

[github-actions]

🟢 No visual differences found

Our visual comparison tests did not find any differences in the UI.

[danielguillan]

LGTM! 👍