Replace the SourceTextModuleRef::GetCell method.

Bug: v8:7790
Change-Id: I65e2f121b9d37c39e5d208d68409f61724ce198e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2410192Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69966}