fix: Add shadow dom support for tab stops visualizations

[sfoslund]

Details

Adds shadow dom support for tab stops visualization, before this change:

[Screenshot of new tab stops shadow doms test resource page with tab stops visualization from current canary extension. There are only 3 tabbed elements visualized and shadow dom elements are skipped.]

After:
[Screenshot of new tab stops shadow doms test resource page with tab stops visualization from built dev extension. There are 5 tabbed elements visualized, including open shadow dom elements.]

Motivation

Addresses issue #5489

Context

First half of #5489 was addressed in #5728, this addresses the remainder.

Pull request checklist

• Addresses an existing issue: Improve shadow DOM support (visualizations & tab stops automation) #5489
• Ran yarn null:autoadd
• Ran yarn fastpass
• Added/updated relevant unit test(s) (and ran yarn test)
• Verified code coverage for the changes made. Check coverage report at: <rootDir>/test-results/unit/coverage
• PR title AND final merge commit title both start with a semantic tag (fix:, chore:, feat(feature-name):, refactor:). See CONTRIBUTING.md.
• (UI changes only) Added screenshots/GIFs to description above
• [n/a] (UI changes only) Verified usability with NVDA/JAWS

[peterdur]

Looks like this may also address #6042. The MSN portal uses lots of shadow DOM.

[waabid]

Is there a particular reason we're using inheritance for this? Do we foresee this being a base class that others would use?

We generally favor using composition over inheritance unless there's a strong reason to use inheritance.

[sfoslund]

I chose inheritance here since it felt like there was a lot of duplicated code between SingleFrameTabStopsListener and TabStopsOrchestrator (which both use this as a base class with these PR changes)- specifically, managing the shadow root event listeners felt like it lends itself more to inheritance.

[sfoslund]

@waabid I'm going to go ahead and merge this since it's been approved and has been sitting for so long, but let me know if you have any further feedback and I'll be happy to make a follow up PR to resolve it!

[peterdur]

A couple of notes, but generally looks good!