Add static asserts that we only use specific types for flag values.
Also, document that string values are not be frozen yet, and add TODOs
to fix that.

R=cbruni@chromium.org

Bug: v8:12887
Change-Id: I7367108810f0c6463509f744c5cefd9392c469fb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3852487Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82714}