[Node.js package] Add TypeScript type definitions

Hello, I added type definitions for this project to https://github.com/DefinitelyTyped/DefinitelyTyped/pull/67445, enabling users to `npm install @types/vosk` into TypeScript applications and getting type definitions through that.

The only thing required here is an approval (or comment of approval) on that pull request from a maintainer of this project. If we can get that in, we can merge this. :)