build: add systemd service

[axelf4]

Adds a systemd unit configuration file to be installed by CMake.

My motivation is being able to auto-start Quickshell on NixOS/niri with just the following module options:

systemd.packages = with pkgs; [ quickshell ];
systemd.user.services.quickshell.wantedBy = [ "niri.service" ];

A caveat is that the service PATH will be limited, meaning DesktopEntry.execute() will often not work directly.

C.f.: https://github.com/Alexays/Waybar/blob/0776e694df56c2c849b682369148210d81324e93/resources/waybar.service.in

Tangentially, it would be neat if an Exec= key got added to org.quickshell.desktop, so it could be placed in /etc/xdg/autostart.

[outfoxxed]

I don't particularly like this because multiple configs are intended to be usable.

[axelf4]

Sure, but it could still be valuable as an example or starting point in that case, while being convenient for anyone who only uses their single own config. But if you disagree we will close this.

[Nicknamely]

I don't particularly like this because multiple configs are intended to be usable.

I tried to run quickshell as a service after migrating to UWSM, but I haven't been able to get it working. The config still runs fine from the CLI. See #166 for more details.