New sizes for Overlay and Dialog

[dgreif]

Introducing new sizes for Overlay because the originals were designed with Dialog in mind. Since Dialog has it's own sizing, Overlay now has the opportunity to support a wider range.

Also switching from sm to small naming convention to match PVC and other places in PRC like Label. Still up in the air on the xLarge values. Should they be xLarge, extraLarge or xl?

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: 5e9a9b2

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

This PR includes changesets to release 1 package

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@primer/components	Minor

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