librewolf: Make mullvad the default search engine

Might add some more search engines later, but Mullvad usually produces
better results for less popular content, especially with topics such as
PinePhone troubleshooting.

home/librewolf.nix

@@ -6,6 +6,21 @@
 
     package = pkgs.librewolf.override { cfg.speechSynthesisSupport = false; };
 
+    profiles.default = {
+      search = {
+        force = true;
+        default = "Mullvad";
+        privateDefault = "Mullvad";
+
+        engines = {
+          Mullvad = {
+            urls = [{ template = "https://leta.mullvad.net/?q={searchTerms}"; }];
+            icon = "${pkgs.mullvad-vpn}/share/icons/hicolor/32x32/apps/mullvad-vpn.png";
+          };
+        };
+      };
+    };
+
     settings = {
       "middlemouse.paste" = false;