fix(ActionList): description font size and truncation

[dgreif]

Fixing a couple small issues with ActionList.Item styles:

• Description font size should be 12px instead of 14px
• Inline descriptions should be truncated with ellipsis, rather than wrapping. I had to redo the flex styles a bit to make the main text area grow properly.

Also added a new story for stress testing these styles

Screenshots

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: eafa1b5

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

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@primer/components	Patch

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