Resolve "panic when entering "a"" ✨️

Closes #71 🐔️👍️
Approved-by: aalbert <augustin.albert@bleu-azure.fr>

🦀️🍰🦀️🍰🦀️🍰

* feat(proost): add test for is_command behavior with 1 letter

* fix(proost): add missing boundary check