Configure 'fourmolu'

[chshersh]

It would be nice to configure an automatic code formatter, close to the existing style, and a CI check.

I'd like to choose fourmolu because it allows configuration and is still actively developed.

• https://github.com/fourmolu/fourmolu

To be specific, the task includes:

• [x] Choosing a proper config as close as possible to the existing code style
• [x] Removing .stylish-haskell config and replacing it with the fourmolu config
• [x] Formatting the entire code
• [x] Adding a CI check to verify if the code is properly formatted
• [x] Documenting style guide and how to run a formatter