Set up DoctestFormatter

[angelikatyborska]

This PR sets up a formatter plugin that deals with doctests, https://hexdocs.pm/doctest_formatter/readme.html

The plugin formats Elixir code in the test according to the rules that apply to the rest of the codebase, and also makes the iex> prompts consistent.

Disclaimer: I'm the author of the plugin, and the plugin was written based on @PragTob's suggestion 🙂

[PragTob]

This is omega nice! Mega Dzieki!

I'm traveling right now, I'll fix the damn macos build and then merge the MR :sweat_smile:

I looked the changes over and it's exactly what I was looking for, this is amazing work thank you very much!

[PragTob]

Thanks a lot @angelikatyborska !