Scripts are treated as web snapshots if they start with a magic number.
This enables end-to-end web snapshot implementations without changing
the embedders.

Bug: v8:11525
Change-Id: Ib8b098bb8cf0b9f96894009414b1cea7646b60dd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3218977Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77389}