In preparation for AllocationObserver changes, move AllocationObserver
related code into its own file.

Bug: v8:10315
Change-Id: I65d5a51662ff192c7b05d4229d8ca27f4a53aa3e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2304580Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68924}