Disallow non-attribute frozen arrays

[domenic]

Also an an example of how to specify frozen array attributes.

Closes #1399.

• [x] At least two implementers are interested (and none opposed):
  • Should be compatible with all code
• [x] Tests are written and can be reviewed and commented upon at:
  • Not needed
• [x] Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/350519230
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1905806
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=276108
  • Deno: N/A
  • Node.js: N/A
  • webidl2.js: https://github.com/w3c/webidl2.js/issues/791
  • widlparser: https://github.com/plinss/widlparser/issues/87
• [x] MDN issue is filed: N/A
• [x] The top of this comment includes a clear commit message to use.

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

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