- Add an inline fast path for flat strings.
- Efficient shape queries.
- Add constants for seq/external one/two-byte string tags.

Bug: v8:12195
Change-Id: Ia2c3d3c9207ab56bc4f0b425d21c7c93ad98aef4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3259530
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77706}