[whatwg/webidl] Introduce three new extended attribute grammars (PR #1503)

These new grammars will be used by the HTML spec for attribute reflection extended attributes.

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You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/webidl/pull/1503

-- Commit Summary --

  * Introduce three new extended attribute grammars

-- File Changes --

    M index.bs (62)

-- Patch Links --

https://github.com/whatwg/webidl/pull/1503.patch
https://github.com/whatwg/webidl/pull/1503.diff

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