Add state-version option

nixosModules/clanCore/state-version/default.nix

@@ -0,0 +1,31 @@
+{
+  lib,
+  config,
+  ...
+}:
+let
+  var = config.clan.core.vars.generators.state-version.files.version or { };
+in
+{
+  options.clan.core.settings.state-version = {
+    enable = lib.mkEnableOption "automatic state-version generation.
+
+    The option will take the specified version, if one is already supplied through
+    the config or generate one if not.
+    ";
+  };
+
+  config = lib.mkIf (config.clan.core.settings.state-version.enable) {
+    system.stateVersion = lib.mkDefault (lib.removeSuffix "\n" var.value);
+    clan.core.vars.generators.state-version = {
+      files.version = {
+        secret = false;
+        value = lib.mkDefault config.system.nixos.release;
+      };
+      runtimeInputs = [ ];
+      script = ''
+        echo -n ${config.system.stateVersion} > "$out"/version
+      '';
+    };
+  };
+}