kitty: make extraConfig obey mkOrder

this change makes kitty.extraConfig obey the lib.mkOrder function so
that text can be inserted at the right place.

tests/modules/programs/kitty/example-mkOrder.nix

@@ -0,0 +1,49 @@
+{ lib, pkgs, ... }:
+{
+  config = {
+    programs.kitty = {
+      enable = true;
+
+      darwinLaunchOptions = lib.mkIf pkgs.stdenv.hostPlatform.isDarwin [
+        "--single-instance"
+        "--directory=/tmp/my-dir"
+        "--listen-on=unix:/tmp/my-socket"
+      ];
+
+      settings = {
+        scrollback_lines = 10000;
+        enable_audio_bell = false;
+        update_check_interval = 0;
+        background_opacity = 0.5;
+      };
+
+      font.name = "DejaVu Sans";
+      font.size = 8;
+
+      keybindings = {
+        "ctrl+c" = "copy_or_interrupt";
+        "ctrl+f>2" = "set_font_size 20";
+      };
+
+      actionAliases = {
+        "launch_tab" = "launch --cwd=current --type=tab";
+        "launch_window" = "launch --cwd=current --type=os-window";
+      };
+
+      environment = {
+        LS_COLORS = "1";
+      };
+
+      extraConfig = lib.mkOrder 535 ''
+        include ~/.cache/wal/colors-kitty.conf
+      '';
+    };
+
+    nmt.script = ''
+      assertFileExists home-files/.config/kitty/kitty.conf
+      assertFileContent \
+        home-files/.config/kitty/kitty.conf \
+        ${./example-mkOrder-expected.conf}
+    '';
+  };
+}