Continue porting darwin, make settei module(s) more reusable by others.

modules/darwin/common/default.nix

@@ -0,0 +1,3 @@
+{
+  system.stateVersion = 4;
+}

modules/darwin/default.nix

@@ -1,5 +1,6 @@
 {config, ...}: {
   flake.darwinModules = {
     settei = import ./settei {inherit (config) perInput;};
+    common = ./common;
   };
 }

modules/darwin/settei/sane-defaults.nix

@@ -11,7 +11,5 @@
     security.pam.enableSudoTouchIdAuth = true;
 
     users.users.${username}.home = "/Users/${username}";
-
-    system.stateVersion = 4;
   };
 }