We should cancel tasks before we destroy objects like the string table
from the isolate. So far our concurrent tasks didn't interact with
the isolate much, but we now have tasks that can e.g. access the heap
concurrently. While this is not strictly necessary at the moment, I want
to see whether there might be problems with that approach. In the future
this order will be required e.g. for baseline batch compilation tasks.

Bug: v8:11708
Change-Id: I80d8a9447bb51e9ae84ed9b6c0da6183290dc70b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3164338
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76989}