Add TypeScript module declaration file (.d.ts)

[Sv443]

This PR adds a .d.ts module declaration file, enabling the usage of this package in TypeScript.

Example images:

• Class I made that wraps around your package (can be found here):
  

• Using my wrapper class:
  

• Code completion of the presence object:
  

• Running that code yields the following RPC:

[devsnek]

I don't maintain types, send this to @types or smth.