documentation-highlighter: Only depend on needed files

Makes the build independency of the default.nix and update.sh file by
explicitly specifying the files that are needed in the result

This allows changing those files without causing a rebuild

pkgs/misc/documentation-highlighter/default.nix

···
     platforms = lib.platforms.all;
     maintainers = [ lib.maintainers.grahamc ];
   };
+  src = lib.sources.cleanSourceWith {
+    src = ./.;
+    filter = path: type: lib.elem path (map toString [
+      ./highlight.pack.js
+      ./LICENSE
+      ./loader.js
+      ./mono-blue.css
+      ./README.md
+    ]);
+  };
 } ''
-  cp -r ${./.} $out
+  cp -r "$src" "$out"
 ''