Add TypeScript type declaration file

[Altareos]

I am currently using this module in a TypeScript project (thank you for your work, btw), but the compiler wouldn't stop complaining about missing types.

Instead of having you rewrite the entire thing in TypeScript (#70), I have created a type declaration file that should be enough for the compiler to understand the types of your classes and functions when used in TS projects.

The comments are directly lifted from your code, with minor modifications.

[kewisch]

Thank you so much! I'll take this for now since some type are better than no types. I was hoping we could use the jsdoc approach so we don't have to maintain separate types. There was some discussion about the future of this project w.r.t. typescript in a separate issue.

[Altareos]

You're very welcome! I'm not a big fan of the duplication either, but I didn't want to impose a TS build step to your publication process (and also I'm still not great with JSDoc). If anyone reading this wants to merge the types and source code, they'll be very welcome to!