Moving from `mjs` to `ts`

[antoinekm]

What did I change? I just switched from mjs to ts (commonjs), no more/no fewer features, all this to get all ts benefits.

Why did I change it? As a new contributor, it is difficult to directly understand how the code is working, using the shared functions and what should it return, etc. Also, I think that in the long term, it might be a good idea to have safe types for future contributions.