version: add module

modules/misc/version.nix

@@ -0,0 +1,24 @@
+{ config, lib, ... }:
+
+with lib;
+
+{
+  options = {
+    home.stateVersion = mkOption {
+      type = types.enum [ "18.09" "19.03" ];
+      default = "18.09";
+      description = ''
+        It is occasionally necessary for Home Manager to change
+        configuration defaults in a way that is incompatible with
+        stateful data. This could, for example, include switching the
+        default data format or location of a file.
+        </para><para>
+        The <emphasis>state version</emphasis> indicates which default
+        settings are in effect and will therefore help avoid breaking
+        program configurations. Switching to a higher state version
+        typically requires performing some manual steps, such as data
+        conversion or moving files.
+      '';
+    };
+  };
+}