Refactor-only: move the more involved EmbeddedFileWriter methods into
the .cc file.

Bug: v8:9103
Change-Id: I546c23544a0425a32cbd04cecc759f9b553b7071
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1624207
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61748}