Moves ReadOnlyPage, ReadOnlyArtifacts, ReadOnlySpace and
SharedReadOnlySpace out of spaces.h and into read-only-spaces.h, as well
as creating a corresponding .cc file.

Bug: v8:10473
Change-Id: I9d8b49d61ed643fd6e16919d571a909ab6fce407
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2171197Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67531}