Ability to group custom fields into its own root directory

[ttbarnes]

At the moment as per the example custom fields, all the fields are at the top level of the app.

It would be great to be able to have these in a custom-fields directory to save littering the root of a project.

Currently this is possible when running in dev mode, but a production build fails due to NextJS/webpack.

Are there plans to support this or perhaps allow for some custom webpack configurations?