This is a reland of commit a1392fa1

The original change was reverted due to v8:13135, which was fixed
separately.

Original change's description:
> Enable background merging when --stress-background-compile
>
> This change adds new functions to BackgroundCompileTask which closely
> match those in BackgroundDeserializeTask. These functions allow a caller
> to manage background merging of newly compiled content into an existing
> Script from the Isolate compilation cache. These functions are not yet
> exposed via the API; instead, StressBackgroundCompileThread uses them to
> increase test coverage of the merging logic.
>
> Bug: v8:12808
> Change-Id: I4d2f429164223785169fe447ce2bdd8beaee00d4
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3793959
> Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
> Reviewed-by: Leszek Swirski <leszeks@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#82121}

Bug: v8:12808
Change-Id: I530c6e87bbad11e178ef4abfdc25fa98530f0224
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3806590Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#82165}