Cleanup and write tests

Hello,

I did a quick walkthrough in this repository and it seems like it's a PoC as there's a lot of dead code, and quick hacks. The repository also is missing unit tests which we'd expect from a library that will be an interface for a payment backend.

There are also some pieces that may cause bugs, for example:

This one, dotenv.config is async and may load env variables anytime, which means that we might not be available when consumed

chargily-epay-js/src/index.ts

Line 8 in 22f3a7a

dotenv.config()

Also, I'm having a hard time understanding the logic, there're some references to axios which means that at some point we'll be calling a backend service, but how and where?