Several ActionMenu a11y fixes

[camertron]

What are you trying to accomplish?

This PR fixes the following ActionMenu issues:

1. Hides the menu when focus leaves the component.
2. Focuses on the first list item when the menu is activated with the mouse.
3. Allows disabling list items while still permitting them to be focused with the keyboard.

This PR also contains a significant refactoring of ActionMenu's event handling logic and test helpers.

Depends on github/clipboard-copy-element#71

Screenshots

Focusing the first item. Notice the focus ring moves to the second item, indicating the first item was previously focused even though the focus ring does not appear.

focus_first_item.mov

Disabled items can be focused but not activated

disabled_items.mov

Menu closes on focus out

focus_out.mov

Integration

No changes necessary in production.

List the issues that this change affects.

Closes https://github.com/github/primer/issues/2661
Closes #2253
Closes https://github.com/github/primer/issues/2421

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: 3d746cd

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	Patch

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[camertron]

NOTE: there will be a few failing system tests until github/clipboard-copy-element#71 is merged, published, and bumped in this PR.

Done

[keithamus]

The refactor makes the changes here a little difficult to follow but considering there are new tests and the existing tests pass, I'll trust that this hasn't regressed any behaviour.