This introduces a new DirectHandle class with a deliberately similar API
to Handle. It uses an API that uses identical method names for symmetry,
but with an address field containing a direct pointer to JS heap objects
(or SMI).

Direct handles are experimental and can be enabled with the
v8_enable_conservative_stack_scanning gn option. The motivation for them
is described in the design doc [1].

[1]: https://docs.google.com/document/d/1uRGYQM76vk1fc_aDqDH3pm2qhaJtnK2oyzeVng4cS6I/

Bug: v8:13270
Change-Id: I0a6e0581adb5fa3b420efec3ba2b6d609d945c52
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3820483
Commit-Queue: Jake Hughes <jh@jakehughes.uk>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83434}