Reaorganize repository tree files about default providers

[brichet]

We should reorganize the repository for a better readability, to have a default-providers directory.
This directory could contain one directory per provider, each including the completer class, the generated settings and the instructions.

- default-providers
  - index.ts (providing the AIProvider[])
  - MistralAI
    - completer.ts
    - instructions.md
    - settings.json
  - OpenAI
    - ...

First expressed at #50 (comment)