Implements match indices for regexp, as specified by
https://github.com/tc39/proposal-regexp-match-indices,
a stage 3 TC39 proposal. This implementation is hidden
behind the '--harmony-regexp-match-indices' flag.

Regexp match indices extends the JSRegExpResult object
with an array of indices of matches, as well as a
dictionary of capture names to match indices.

Bug: v8:9548
Change-Id: I9866a2d1f5af6a507de710357cb5e74c694e7558
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1734937
Commit-Queue: Joshua Litt <joshualitt@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63272}