Introduce SourceTextModule as a subclass of Module.  Move all the
JavaScript-module-specific code down from Module to
SourceTextModule, with all code applicable to other future
module types remaining in Module.

With this change, Module is roughly equivalent to the spec's
Abstract Module Record and SourceTextModule is roughly equivalent
to Source Text Module Record.

Bug: v8:9292
Change-Id: I6e9cd3ece9d0c1da57e52f8af8ed5848d87dd22d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1633154
Commit-Queue: Dan Clark <daniec@microsoft.com>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62296}