Instead of implementing ExternalObjects as plain JSObjects with a single
EmbedderDataSlot pointing to a Foreign containing the actual raw
pointer, this CL now creates a new JSExternalObject type that directly
contains the external pointer.

As a side-effect of this refactoring, nullptr values are now no longer
valid for ExternalObjects.

Change-Id: Ic8ff334681c966e823ca70f34dd1efaaa21a0789
Cq-Include-Trybots: luci.v8.try:v8_linux64_heap_sandbox_dbg_ng,v8_linux_arm64_sim_heap_sandbox_dbg_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3513234Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79459}