Add position to the return of %SegmentIterator%.next()
which newly added to the spec in
https://github.com/tc39/proposal-intl-segmenter/pull/42

Bug: v8:8305
Cq-Include-Trybots: luci.v8.try:v8_linux_noi18n_rel_ng
Change-Id: I8de7102acb670a6c529ab3e35601c78a8dc7703c
Reviewed-on: https://chromium-review.googlesource.com/c/1278636Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Daniel Ehrenberg <littledan@chromium.org>
Commit-Queue: Frank Tang <ftang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56963}