Upgrading rails/webpacker v3.5 to v4

The following steps can be followed to update a Webpacker v3.5 app to v4.

1. Update the gem webpacker and the package @rails/webpacker
2. Merge changes from the new default .babelrc to your /.babelrc. If you are using React, you need to add "@babel/preset-react", to the list of presets.
3. Copy the file .browserslistrc to /.
4. Merge any differences between config/webpacker.yml and your /config/webpacker.yml.

Here is an example commit of these changes.