Transpile to CommonJS format

[coryhouse]

The latest version of create-react-app disables transpiling to CommonJS. CommonJS is the current standard format for Node modules. So we need to manually enable this so that our library is published in CommonJS format.

Here's the necessary code change.

I've updated this repository to reflect the change. I'm working on an update to the relevant video in the course, and will close this issue when complete.

[coryhouse]

Closing since the course, exercise, and this repo have been patched to resolve.