feat(css): Add HTML focusgroup global attribute

[ramiy]

Summary

The HTML focusgroup global attribute is used to designate an element as a focus group container. This allows related focusable elements within the group to behave as a single, logical focus navigation group, improving keyboard accessibility. The attribute helps manage focus traversal in complex user interfaces, ensuring that navigation within grouped elements is intuitive and follows a set structure.

Spec/Explainer:

• https://open-ui.org/components/focusgroup.explainer/
• https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/Focusgroup/explainer.md

Chrome:

• https://chromestatus.com/feature/5637601087193088
• https://issues.chromium.org/issues/40210717

[queengooborg]

Thanks for digging into this -- however, it looks like this was only featured in a small number of Chrome releases, and removed in a release that is almost two months old. Per our guidelines, we would just end up removing this in a month anyways, so I'm going to close this PR. Thank you anyways!