Prioritize ES5 (main) over ES6 (module) in webpack resolve.mainFields config [build failures are irrelevant]

Created by: jedwards1211

Since Webpack by default loads from module instead of main in package.json, any library that publishes both ES5 and ES6 this way will trigger minification errors with create-react-app. But if we change the Webpack config to try main first, libraries that publish both ES5 and ES6 won't have this problem, and create-react-app will still do its best on libraries that only have ES6.

I'm really surprised no one has made this change already. Others have suggested this change yet I thought of it independently, so I would think it's obvious.

Here's how I came upon this, devised my fix and verified that it works:

1. A user of my material-ui-render-props-styles package, which publishes both ES5 and ES6, reported problems with create-react-app and gave me an example repo: https://github.com/solayao/dizzy_comic_fe
2. I cloned that repo and installed deps in it, confirmed yarn build has the minification error
3. Then I ran npm edit react-scripts (I tried linking in my fork, but a few too many things had changed from the version the project was using)
4. I added mainFields: ['browser', 'main', 'module'] to its webpack.config.prod.js resolve configuration
5. retried yarn build -- it worked!