To reduce gn check errors, this moves EmbeddedFileWriterInterface into
its own header file that can be included directly by isolate.cc since
embedded-file-writer.h is in the mksnapshot target.

Bug: v8:7330
Change-Id: I3bfb1f48c646680b71189e8229b8df54ead9eea0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2727817
Auto-Submit: Dan Elphick <delphick@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73118}