style: Make dummy export really export nothing

I vaguely remember seeing this trick in a GitHub issue thread, but I
can't recall where. A bit of searching gave me this:

  microsoft/TypeScript#25386 (comment)

...but I cannot verify that @Andy-MS came up with this trick.

Author: pastelmind

src/types.js

@@ -13,4 +13,4 @@
  */
 
 // Dummy export to make TypeScript treat this file as a module.
-export default undefined;
+export {};