[heycam/webidl] Add [Historical] extended attribute (#929)

Closes #910 

This intentionally applies it to member constructs rather than interfaces to keep it simple.
You can view, comment on, or merge this pull request online at:

  https://github.com/heycam/webidl/pull/929

-- Commit Summary --

  * Add [Historical] extended attribute

-- File Changes --

    M index.bs (25)

-- Patch Links --

https://github.com/heycam/webidl/pull/929.patch
https://github.com/heycam/webidl/pull/929.diff

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https://github.com/heycam/webidl/pull/929