Add type declarations

[gnugomez]

Is your feature request related to a problem? Please describe. No, in fact, I would like to use this library on a ts project, but without the type definitions, the code is so wired.

Describe the solution you'd like This project is built in js so I will understand if this couldn't be possible. But I think it will be a great idea to implement it for future users who will want to use ts.