This interface is meant to eventually replace the existing
v8::PageAllocator interface. Beyond general refactoring of the
PageAllocator APIs, the new interface now supports the concept of
(contiguous) address space reservations, which previously had to be
implemented through page allocations. These reservations now make better
use of provided OS primitives on Fuchsia (VMARs) and Windows
(placeholder mappings) and can be used to back many of the cages and
virtual memory regions that V8 creates.

The new interface is not yet stable and may change at any time without
deprecating the old version first.

Bug: chromium:1218005
Change-Id: I295253c42e04cf311393c5dab9f8c06bd7451ce3
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/+/3301475
Commit-Queue: Samuel Groß <saelo@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78235}