kitty: allow not setting shell_integration

This allows better support for using kitty's built-in shell integration
method by setting `shellIntegration.mode' to `null', which makes it not
be set at all in the config file and disables modification to shell
initialization scripts by default. If this is used, the user should set
`settings.shell_integration' instead (if shell integration is desired),
which receives no special handling.

tests/modules/programs/kitty/null-shellIntegration.nix

@@ -0,0 +1,22 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  config = {
+    programs.kitty = {
+      enable = true;
+
+      shellIntegration.mode = null;
+    };
+
+    test.stubs.kitty = { };
+
+    nmt.script = ''
+      assertFileExists home-files/.config/kitty/kitty.conf
+      assertFileContent \
+        home-files/.config/kitty/kitty.conf \
+        ${./null-shellIntegration-expected.conf}
+    '';
+  };
+}