Now that we support extension updates, it may be useful to list
pending updates from the command line. It's easy enough to support,
so add a corresponding option to the list command.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/988
The gnome-extensions tool code is really independent from the rest of the
code base, and could be used either as part of the gnome-shell build or as
stand-alone project (for example for the extension-ci docker image).
We can actually support both cases by moving the code to a subproject.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/877
