NixOs module: add option to use UWSM

[meck]

This is mostly adapted from the equivelent nixpgks hyprland option.

USWM gives better options for delaying start of dependent systemd units (waybar and such). And handles putting processes in slices.

[Brisingr05]

handles putting processes in slices.

FYI niri started using niri-session already does this for programs started using spawn-at-startup in the config, niri keybinds, or niri msg action spawn, but yes, UWSM definitely gives more flexibility.