[FEATURE] Add support for .path units

[nil0x42]

Hi ! I liked the principle of using systemd as backend for workflow scheduling, while providing a simpler interface.

One feature i would like very much is ability to have jobs triggered on path change instead of timer.
job()'s first argument could support additional syntax such as onPathChange=/etc/passwd for example, and then geerate appropriate dependency with generated systemd .path unit

[karlicoss]

Hi! Glad you liked it :)
Yeah, interesting idea -- I haven't much application for it yet though

Probably would require changes

• here to handle path argument(e.g. when_path_changed)

  dron/dron.py

  Lines 432 to 436 in 4b7653d

  	when = j.when
  	if when is None:
  	continue
  	t = timer(unit_name=uname, when=when)
  	yield check(uname + '.timer', t)

• might need changes around

  dron/dron.py

  Line 551 in 4b7653d

  if unit.endswith('.timer'):

  and

  dron/dron.py

  Line 575 in 4b7653d

  elif unit.endswith('.timer'):

• and the dron monitor command is has a hard assumption of each unit having a correspondent timer I think -- might be easy to fix by filtering out the ones without timers, although the proper fix would probably be to also display the ones with paths dependencies properly

  dron/dron.py

  Line 838 in 4b7653d

  def _cmd_monitor(managed: State, *, params: MonParams):