Action menu avatar item

[camertron]

What are you trying to accomplish?

This PR implements the following accessibility fixes for the ActionMenu component:

1. the component should explicitly disallow passing an aria-label to leading icons.
2. the component should disallow providing alt text to avatars.
3. the component should support a special type of list item for avatars that enforces a) no alt text, and b) ensures the username appears as the item text.

Screenshots

No visual changes

Integration

Yes. Several usages of NavList will need to stop calling #with_leading_visual_avatar in favor of #with_avatar_item.

List the issues that this change affects.

Fixes #https://github.com/github/primer/issues/2246

Risk Assessment

• Low risk the change is small, highly observable, and easily rolled back.
• Medium risk changes that are isolated, reduced in scope or could impact few users. The change will not impact library availability.
• High risk changes are those that could impact customers and SLOs, low or no test coverage, low observability, or slow to rollback.

Accessibility

• No new axe scan violation - This change does not introduce any new axe scan violations.

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Lookbook)
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge

[changeset-bot]

🦋 Changeset detected

Latest commit: 18f508e

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

This PR includes changesets to release 1 package

Name	Type
@primer/view-components	Minor

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[github-actions]

⚠️ Visual differences found

Our visual comparison tests found UI differences. Please review the differences by viewing the files changed tab to ensure that the changes were intentional.

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[jonrohan]

Nice refactor 👍🏻