Introduce attribute reflection extended attributes

[lukewarlow]

A number of extended attributes are added which trigger IDL attribute reflection:

• [Reflect] and [Reflect=string] cover basic content attribute reflection cases.

• [ReflectSetter] and [ReflectSetter=string] covers content attribute reflection only for setters.

• [ReflectURL] and [ReflectURL=string] covers reflection of USV attributes, which represent URLs.

• [ReflectNonNegative] and [ReflectNonNegative=string] covers reflection of long attributes that should be limited to only non-negative numbers.

• [ReflectPositive] and [ReflectPositive=string] covers reflection of double and unsigned long attributes which should be limited to positive numbers.

• [ReflectPositiveWithFallback] and [ReflectPositiveWithFallback=string] covers reflection of double and unsigned long attributes which should be limited to positive numbers with fallback.

Additionally:

• [ReflectRange=(integer, integer)] can be used alongside others on unsigned long attributes that should be clamped to a range.

• [ReflectDefault=number] can be used alongside others on double, long and unsigned long attributes that should be reflected with a default value.

Depends on: whatwg/webidl#1503

• At least two implementers are interested (and none opposed):
  • N/A, because it's editorial
• Tests are written and can be reviewed and commented upon at:
  • N/A
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/u/1/issues/434737815
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1979778
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=296589
  • Servo: Implement standardised attribute reflection WebIDL extended attributes servo/servo#38317
  • Ladybird: Implement standardised attribute reflection WebIDL extended attributes LadybirdBrowser/ladybird#5631
  • JSDOM: Implement [ReflectSetter] IDL extended attribute jsdom/jsdom#3913
• 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 #3238

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

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