[builtins] Adjust String.prototype.matchAll as per spec changes (https://github.com/tc39/proposal-string-matchall/pull/38)

- Removes IsRegExp check and special handling when false
- Removes MatchAllIterator
- Extracts previously inlined CreateRegExpStringIterator
- Update comments to match spec text and numbering

Bug: v8:6890
Change-Id: Ie81757a499acc77910f029835fb042e70d86d83d
Reviewed-on: https://chromium-review.googlesource.com/c/1317830
Commit-Queue: Peter Wong <peter.wm.wong@gmail.com>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57488}