gtksourceview4: Backport Nix syntax highlighting support

This brings the support to editors that haven't ported to GTK4.

I am looking at https://gitlab.gnome.org/GNOME/gtksourceview/-/tree/master/data/language-specs
and most files are not changed for six years so I assume no breaking changes for these specs
are made so far.

pkgs/development/libraries/gtksourceview/4.x.nix

@@ -1,6 +1,7 @@
 { lib
 , stdenv
 , fetchurl
+, fetchpatch2
 , meson
 , ninja
 , pkg-config
@@ -40,6 +41,19 @@ stdenv.mkDerivation (finalAttrs: {
     # 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
+
+    # nix.lang: Add Nix syntax highlighting
+    # https://gitlab.gnome.org/GNOME/gtksourceview/-/merge_requests/303
+    (fetchpatch2 {
+      url = "https://gitlab.gnome.org/GNOME/gtksourceview/-/commit/685b3bd08869c2aefe33fad696a7f5f2dc831016.patch";
+      hash = "sha256-yeYXJ2l/QS857C4UXOnMFyh0JsptA0TQt0lfD7wN5ic=";
+    })
+
+    # nix.lang: fix section name
+    (fetchpatch2 {
+      url = "https://gitlab.gnome.org/GNOME/gtksourceview/-/commit/1dbbb01da98140e0b2d5d0c6c2df29247650ed83.patch";
+      hash = "sha256-6HxLKQyI5DDvmKhmldQlwVPV62RfFa2gwWbcHA2cICs=";
+    })
   ];
 
   nativeBuildInputs = [