Add logic to update radio button group checkedness when an input radio becomes connected

[EdgarChen]

Closes #10897.

• At least two implementers are interested (and none opposed):
  • Chromium and WebKit already behave like this.
• Tests are written and can be reviewed and commented upon at:
  • [Gecko Bug 1938489] Don’t update other radio inputs in the same radio group when a radio input is appended to a disconnected subtree web-platform-tests/wpt#50048
• Implementation bugs are filed:
  • Chromium: None
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1938489
  • WebKit: None
  • Deno (only for timers, structured clone, base64 utils, channel messaging, module resolution, web workers, and web storage): None
  • Node.js (only for timers, structured clone, base64 utils, channel messaging, and module resolution): None
• Corresponding HTML AAM & ARIA in HTML issues & PRs: None
• MDN issue is filed: None
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

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/input.html ( diff )

[EdgarChen]

cc @domfarolino @annevk