`poetry exit` to safely exit the virtualenv

Add a poetry exit command to exit a poetry shell; do so safely when the poetry shell is not active already. Since exit will exit a bash shell when a poetry shell is not active, it’s “dangerous”. At present, there are issues asking for some form of “marker” that a poetry shell is active (i.e. there seems to be no easy way to tell whether or not a poetry shell is active without issuing another poetry shell to get poetry feedback about it). A poetry exit command is an additional feature that provides a safe exit, regardless of whether a poetry shell is active or if the status of it is visible.