Multiple assignment list unwrapping

[dlrobertson]

Summary

Similar to #508, it would be nice to be able to assign multiple variables from a list in a single unwrapping statement.

Example

strictly split returns a list, but in cases like validate and extract, populating local variables with indexing leads to text that is a bit verbose. We currently have something like the following steps:

1. Let splitResult be the result of running strictly split given qualifiedName and U+003A (:).
2. Set prefix to splitResult[0].
3. Set localName to splitResult[1].

With something like #508 for list we could simplify these three steps into something like the following:

1. Set « prefix, localName » to the result of running strictly split given qualifiedName and U+003A (:).

Context

Originally posted by @annevk in #508 (comment)

I think in particular for strictly split it would be helpful if this worked on list-likes as well (the let-side could still use the tuple syntax I suppose), but maybe we ought to tackle that separately.

Also related to whatwg/dom#1134.