In follow-up work, the parser will be refactored to take the input as
raw char arrays instead of a FlatStringReader s.t. it can be reused by
the V8 parser (which has AstRawStrings instead of Strings).

Bug: v8:896
Change-Id: I0e0bda4b34bc23b8bc427ddf3f9516081c42bb8a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3099947Reviewed-by: Patrick Thier <pthier@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76359}