refactor(modules/system): move system config to a module

modules/nixos/system/default.nix

@@ -0,0 +1,16 @@
+{ config, lib, ...}:
+{
+  imports = [
+    ./users.nix
+    ./localisation.nix
+    ./nix-config.nix
+  ];
+
+  options.system = {
+   mainUser = lib.pantheon.mkStrOption;
+  };
+
+  config = {
+    system.stateVersion = "25.05"; # Did you read the comment?
+  };
+}