Whether we install bash-completion support currently depends on whether
the corresponding pkg-config dependency is found.
Turning this into a feature option keeps that behavior by default, but
also allows to explicitly enable or disable the support.
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1072
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
