[heap] Split string table clearing in atomic GC pause

Split string table clearing into multiple parallel chunks for better
parallelization during GC clearing phase. We have seen Jetstream3
runs where this phase alone took over 6ms. It is also generally the
most expensive phase within ClearNonLiveReferences().

Bug: 463528049
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_isolates_rel,v8_linux64_tsan_no_cm_rel,v8_numfuzz_tsan_rel,v8_linux64_tsan_dbg
Change-Id: Iefe9f28f52dfc3b0ed598c01eb9ea80ac9ff4897
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/7523526
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#104964}