Convert CommonJS exports to ES6-compatible exports

We use `export default` begrudgingly here. Ideally we'd use just `export`, though this entire SDK expects things to be exported as a default. Instead of breaking everything, we'll sacrifice our export pattern for a smaller diff - a later commit can always do the default export -> regular export conversion.

src/Skinner.js

@@ -106,5 +106,5 @@ class Skinner {
 if (global.mxSkinner === undefined) {
     global.mxSkinner = new Skinner();
 }
-module.exports = global.mxSkinner;
+export default global.mxSkinner;