Resolve "Bug(Parser): no error when defining a declaration with the same name as a definition" ✨️

Closes #81 🐔️👍️
Approved-by: loutr <l.ta-ma@proton.me>
Approved-by: v-lafeychine <vincent.lafeychine@proton.me>

🦀️🍰🦀️🍰🦀️🍰

* fix(proost): return a `BoundVariable` error when defining a declaration and a definition with the same name