chore: add types for b4a

[achou11]

Towards #14

• Only adds definitions for the Node implementation, not the browser one, which seems out of scope (interested to have them but don't know how to create the separation properly in a declaration file, if possible)

[achou11]

For reference, maybe this is how you can declare for different environments? https://github.com/speakeasyjs/base32.js/blob/master/index.d.ts