[heycam/webidl] Introduce the observable array type (proxy-based) (#840)

Part of #796.

This is intended as a better version of https://github.com/heycam/webidl/pull/836 that (on a spec level) uses proxy traps, instead of Web IDL legacy platform objects, to fix the issues noted in https://github.com/heycam/webidl/issues/796#issuecomment-578273849.

This also includes a second commit which uses the suggestions from @bzbarsky in https://github.com/heycam/webidl/pull/836#issuecomment-580977516 to avoid the resulting array having holes. This seems like a good thing in various ways, although I am a little unsure whether I successfully pulled it off.

This pull request also changes how spec authors are expected to interface with the observable array type, per some of the discussions in #836. I will do a pull request to the constructible stylesheets repository to see how well these work...
You can view, comment on, or merge this pull request online at:

  https://github.com/heycam/webidl/pull/840

-- Commit Summary --

  * Introduce the observable array type (proxy-based)
  * Ensure there are no holes in the array

-- File Changes --

    M index.bs (321)

-- Patch Links --

https://github.com/heycam/webidl/pull/840.patch
https://github.com/heycam/webidl/pull/840.diff

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