This simplifies various bits of logic around EmbedderDataSlots as the
raw part will now always contain a valid index into an external pointer
table entry.

This CL also unifies the initialization of EmbedderDataSlots by
providing a EmbedderDataSlots::Initialize method and adds more
documentation about the layout of EmbedderDataSlots in the different
configurations.

Bug: v8:10391
Change-Id: Ie952598898a7a6c9d40b28d3a7370bfc1291bcf0
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/+/3472495Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79384}