feat: extract some config into modules

modules/nixos/printing.nix

@@ -0,0 +1,21 @@
+{
+  lib,
+  config,
+  ...
+}:
+{
+  options = {
+    printing = {
+      enable = lib.mkEnableOption "CUPS printing with Avahi discovery";
+    };
+  };
+
+  config = lib.mkIf config.printing.enable {
+    services.printing.enable = true;
+    services.avahi = {
+      enable = true;
+      nssmdns4 = true;
+      openFirewall = true;
+    };
+  };
+}