Background styles for focused action list item

[dgreif]

Work in #1135 revealed that ActionListItems were not focusable, and when made focusable, their styling was not correct.

• Make list items focusable by setting tabindex="-1". useFocusZone will latch onto this and set the tab index to 0 when enabled.
• Add background colors/remove outline when items are focused.
• Added focus zone to the ActionList story to make focus state easier to test

Closes #1180

Screenshots

Before

Gray is hover, outline is focused

After

Lighter is hover, darker is focus

Dark Mode

Dark Dimmed Mode

Merge checklist

• Added/updated tests
• Added/updated documentation
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge

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🦋 Changeset detected

Latest commit: 2647438

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

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@primer/components	Patch

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