wesnoth: add update script

pkgs/games/wesnoth/default.nix

···
   lua,
   curl,
   fetchpatch,
+  nix-update-script,
 }:
 
 stdenv.mkDerivation rec {
···
     EOF
     chmod +x "$out/bin/wesnoth"
   '';
+
+  passthru.updateScript = nix-update-script {
+    extraArgs = [
+      "--version-regex"
+      # the minor release number also denotes if this is a beta release:
+      # even is stable, odd is beta
+      "^(\\d+\\.\\d*[02468]\\.\\d+)$"
+    ];
+  };
 
   meta = with lib; {
     description = "Battle for Wesnoth, a free, turn-based strategy game with a fantasy theme";