Support systemd socket activation

[valpackett]

With socket activation, systemd can start the service when it's being used (and let it shut down when unused).

Example configuration:

~/.config/systemd/user/arrpc.socket

[Unit]
Description=arRPC Discord RPC bridge socket
Documentation=https://arrpc.openasar.dev/

[Socket]
ListenStream=127.0.0.1:1337
NoDelay=true

[Install]
WantedBy=sockets.target

~/.config/systemd/user/arrpc.service

[Unit]
Description=arRPC Discord RPC bridge
Documentation=https://arrpc.openasar.dev/

[Service]
#ExecStart=node %h/src/arrpc/src # (<<<use this instead of the below line with a path to a local git checkout, while the patch is not released yet)
ExecStart=npx arrpc
SendSIGHUP=true
Restart=on-failure

systemctl --user daemon-reload
systemctl --user enable --now arrpc.socket

(the units from discord-flatpak-rpc-bridge can be put into ~/.config/systemd/user/ as well btw)