Make the muted content attribute a fallback

[annevk]

And align the muted getter with the :muted pseudo-class.

This also cleans up how we organize all the muted state. This removes the reliance on "creation-time" of the element which is completely bogus as whenever an element is created it has exactly 0 attributes.

Fixes #5013.

• At least two implementers are interested (and none opposed):
  • WebKit
  • Gecko
• Tests are written and can be reviewed and commented upon at:
  • HTML: make muted attribute no longer rely on non-existing element creation-time web-platform-tests/wpt#59322
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/509570175
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=2037015
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=314061
• Corresponding HTML AAM & ARIA in HTML issues & PRs: N/A
• MDN issue is filed: N/A
• The top of this comment includes a clear commit message to use.

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

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