Draft: main/pmos-ui-phosh: use superd for user services

I would like to get feedback on this series, since there are a few things that can probably be improved on:

• ui-phosh package depends on this config, and uses an env var to configure gnome-session to start superd (see the commit message for details)
• the librem 5 package was updated to show how device packages can support starting default user services in specific situations (in this case it's for the device's phosh subpackage)