The is_shared bit bumps the number of reserved bits for Strings'
InstanceType from 6 to 7. This has the side effect of shuffling the
InstanceType enum values.

There are no users of this bit yet. This is steps 1-2 from the following
design doc [1], in preparation for sharing internalized and
in-place-internalizable strings.

[1] https://docs.google.com/document/d/1c5i8f2EfKIQygGZ23hNiGxouvRISjUMnJjNsOodj6z0/edit?usp=sharing

Bug: v8:12007
Change-Id: Idf11a6035305f0375b4f824ffd32a64f6b5b043b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3266017
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77831}