[heycam/webidl] Import (and fix) the promises guide (#490)

Most of the algorithms in https://www.w3.org/2001/tag/doc/promises-guide for resolving and rejecting should be part of IDL (and formalized in the process). Resolving from "in parallel" should be something that HTML defines I think.

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https://github.com/heycam/webidl/issues/490