The template used when creating a new extension is intentionally
minimal, as the sample code in the old extensions-tool proved to
be annoying more often than not.
However as we support per-command options, we don't have to limit
ourselves to a single template, and can offer alternatives for
common use cases.
To prepare for that, namespace the existing template by moving it
into a subfolder.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/812
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
