ci: Update JS tooling

Provided we use the correct version of the SpiderMonkey shell, we
can perform checks using the same engine that is used by gjs.

However some engine features are opt-in, so the set of features enabled
by gjs and js102 may differ. The obvious option for avoiding this is
replacing js102 with gjs for tests.

For that reason the check-potfiles.js script has been ported to gjs
and a second (simpler) script to replace `js102 --compileonly` was
added. This work happened in a separate repository to make sharing
between different JS-based projects easier.

Update the CI image to pull in those scripts and include gjs.

This will also address the issue that `js102 --compileonly` does not
handle modules.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2193>