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
