A dispatcher that currently does nothing, installed on the Isolate.

The implementation is close to the baseline concurrent compiler; but
boilerplate is short enough that a common base class is not worth the
additional complexity.

Bug: v8:7700
Change-Id: Ia34781e24cb6b1f72e5560fb0afe107bb0486092
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3497690
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79353}