gtksourceview5: Add Nix syntax highlighting

So that the default text editor (GNOME Text Editor) can highlight NixOS configuration out of the box.

pkgs/development/libraries/gtksourceview/5.x.nix

@@ -1,6 +1,7 @@
 { lib
 , stdenv
 , fetchurl
+, fetchpatch2
 , meson
 , ninja
 , pkg-config
@@ -37,6 +38,13 @@ stdenv.mkDerivation rec {
     # but not from its own datadr (it assumes it will be in XDG_DATA_DIRS).
     # Since this is not generally true with Nix, let’s add $out/share unconditionally.
     ./4.x-nix_share_path.patch
+
+    # Add Nix syntax highlighting.
+    # https://gitlab.gnome.org/GNOME/gtksourceview/-/merge_requests/303
+    (fetchpatch2 {
+      url = "https://gitlab.gnome.org/GNOME/gtksourceview/-/commit/2cc7fd079f9fc8b593c727c68a2c783c82299562.patch";
+      sha256 = "bTYWjEDpdbnUxcYNKl2YtSLfYlMfcbQSSYQjhixOGS8=";
+    })
   ];
 
   nativeBuildInputs = [