When supervisor is set to run indefinitely, have it (optionally?) shut down after all sessions have exited #21
Description
Currently, if you run Kallichore without an idle timeout, it will run forever. This might not be desirable in some configurations. For example, on Positron Desktop, once you configure the supervisor to let kernels run indefinitely, you can no longer get rid of the supervisor process without some sort of drastic action (running the 'restart kernel supervisor' command, reboot, etc.).
If Kallichore is started without an idle timeout, it should still exit by default if it isn't running anything. In some configurations (e.g. running a remote server) it might be better to let it run -- we could add a command line option for this? --- but in most cases exiting is preferable.