Allow activation element in shadow DOM to be auto-focused after trap is deactivated

[marioliu-block]

Kind of related to #1072 / #1068

I noticed that support for shadow DOM focusing was added for custom elements within focus traps, but not for custom activation elements outside of focus traps.

This PR adds support for focusing on custom activation elements within the default deactivation behavior of the focus trap.

I tried writing Cypress tests for this change, but it doesn't seem to like custom elements and can't seem to focus on them.

PR Checklist

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• Issue being fixed is referenced.
• Source changes maintain stated browser compatibility.
• Web APIs introduced have deep browser coverage, including Safari (often very late to adopt new APIs).
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[changeset-bot]

🦋 Changeset detected

Latest commit: 7530b41

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

This PR includes changesets to release 1 package

Name	Type
focus-trap	Patch

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[stefcameron]

@marioliu-block Thank you for this great PR! You followed existing patterns really well which made reviewing it a breeze. 😄 I just have one tiny request if you don't mind, and then I'll happily merge this and get it published.

[stefcameron]

@all-contributors add @marioliu-block for code, bug

[allcontributors]

@stefcameron

I've put up a pull request to add @marioliu-block! 🎉

[stefcameron]

@marioliu-block Thank you for the update! Merging.