feat: add sleep_monitor command with wake-on-input

[StaszeKrk]

Closes #780.

Adds a sleep_monitor command that turns off a specific monitor and
automatically re-enables it on the next keypress or mouse movement.

Unlike disable_monitor, no explicit enable_monitor call is needed — the
monitor wakes itself on any input event.

Usage:
sleep_monitor,eDP-1

Implementation:

• Adds wake_on_input flag to struct Monitor
• sleep_monitor() disables the output and sets the flag
• wakesleepingmons() is called in keypress, buttonpress, and motionnotify
  to re-enable any flagged monitors on the next real input event

[DreamMaoMao]

I think the code for disabling monitor can be reused. Just need a option