chore: Globally handle focus-visible management for application spanning multiple iframes

[avinashbot]

Issue #, if available: AWSUI-60845

Description of changes: Supporting the niche but real use case where components are rendered into a same-origin iframe (typically using a React portal). We keep the reference counting logic, but extend it so that each frame (including the main one) has its own count. This should release first - the matching PR for the components package will follow.

Check my dev pipeline (AwsUi-v3-dwaraa) to see what it looks like.

This is a stopgap to resolve this issue so that the CSS focus-visible migration isn't tied to this simple bugfix. The "default keyboard" approach of the browser behavior compared to the "default mouse" approach of the JS implementation means that there are some async or initial page load scenarios where the focus ring appears where it previously didn't. Some cases may need fixing, some cases we can live with, but that investigation is a separate topic.

Also, as you can tell from the tests, this only works if Cloudscape components are mounted in all the frames where detection needs to happen. Maybe there's a better approach, but at the same time, it also makes sense that I shouldn't start digging up into parent iframes to listen to events if the application isn't even rendered there.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.39%. Comparing base (ed3ca20) to head (5189c31).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #143      +/-   ##
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+ Coverage   99.36%   99.39%   +0.03%     
==========================================
  Files          33       33              
  Lines         787      833      +46     
  Branches      214      234      +20     
==========================================
+ Hits          782      828      +46     
  Misses          5        5              

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[pan-kot]

Great tests!