[focusgroup] Early Draft to add the focusgroup attribute

[janewman]

• Explainer on OpenUI
• whatwg issue

This is an early draft to add the focusgroup attribute to allow for declarative focus navigation using the arrow keys.
This specs the behavior in the explainer, including the following changes that have not yet been merged:

• [focusgroup] Add sections on scrolling and keyboard conflicts openui/open-ui#1289
• [focusgroup] Clarify tab behavior when focusgroup is bisected by an opted-out item openui/open-ui#1290

• At least two implementers are interested (and none opposed):
  • …
  • …
• Tests are written and can be reviewed and commented upon at:
  • …
• Implementation bugs are filed:
  • Chromium
  • Gecko: …
  • WebKit: …
• Corresponding HTML AAM & ARIA in HTML issues & PRs:
• MDN issue is filed: …
• The top of this comment includes a clear commit message to use.