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[styleguide/web] Add ES7/8 features.

This CL adds ES7/8 features to the ES styleguide, and renames it es.md in light
of its more generic nature.

All features are To Be Discussed except Async Functions which are allowed.

Bug: 671426, 905927
Change-Id: Ib01a7dd3bb5dbcbccf8c262c5210614a4d397594
Reviewed-on: https://chromium-review.googlesource.com/c/1339087
Commit-Queue: calamity <calamity@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609558}