add systemd support (#1253)

* add systemd support
motivation for this is is proper ordering of related/bound/required
services to Hyprland (e.g. swaybg) that would need to have a compositor
ready.

this could possibly be a build-time option of course.

see also:
example/ files for example of services

Signed-off-by: Paymon MARANDI <darwinskernel@gmail.com>

* nix: add withSystemd flag

Signed-off-by: Paymon MARANDI <darwinskernel@gmail.com>
Co-authored-by: Mihai Fufezan <fufexan@protonmail.com>
Co-authored-by: Vaxerski <vaxry@vaxry.net>

example/swaybg@.service

@@ -0,0 +1,13 @@
+; a primitive systemd --user example
+; see example/hyprland.service for more details
+[Unit]
+Description = %p
+BindsTo     = hyprland.service
+Wants       = hyprland.service
+After       = hyprland.service
+
+[Service]
+ExecStart = /usr/bin/swaybg --color #%i
+
+[Install]
+WantedBy = default.target