The new command will take an extension directory set up
with `meson-build --sysext` inside a toolbox, and turn
it into a merged system extension on the host.
Namely it will
1. copy the extension directory from the container to the host
2. set up the extension-release file expected by systemd-sysext
3. compile gsettings schemas if necessary (using the merged
schemas directory)
4. install the extension to either runtime or system
5. merge the extension
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3498>
