[WIP] Add react.config.js

Created by: yordis

A combination of webpack-chain and react.config.js would allow us to extend CRA without having to eject.

// react.config.js

module.exports = function() {
  return {
    webpackOverride(config) {
        // ... use webpack-chain API
    }
  }
}

I would love to have a conversation about this topic since we were asking for years about this and many people could take advantage of this feature.

Or

We could expose a webpack-chain config object as well as the existing plain object. The reasoning is that webpack-merge could do that much and webpack-chain configurations allow us to customize the config easier and more robust.

This will be useful for those who want to use CRA configuration since there is a lot of knowledge on the configuration but are forced to eject.