Add typescript types

[sleeyax]

There's a community maintained @types/stremio-addon-sdk package but its outdated and a PITA to maintain. It would be a big improvement to the developer experience if the types were included directly in this repository.

A full rewrite to TypeScript is not necessary: we can simply improve/add documentation throughout the code base using JSDoc syntax and generate the types automatically. It's a win-win for both JS and TS developers.