Launch Desktop Entries (or arbitrary commands) as Systemd user units, and do it fast.

Based heavily on mechanisms and optimizations introduced into xdg-terminal-exec, it performs function similar to (and behaves similarly to) uwsm's app subcommand, but without the costly startup of Python interpreter or the necessity of having a daemon running for speeding things up. If run on a fast shell (such as dash) with system stored on an SSD, overhead can be as short as ~0.03s, with systemd-run giving additional ~0.03s.