refactor: Added TypeScript Type Definitions

[Aarebecca]

I noticed that the project was missing the ts type definition, so I inferred the type based on the implementation. If you have any questions, please point them out.

[vasturiano]

@Aarebecca that's good, thanks for your effort here, and for opening the PR.

I do think it would be better to base the type definitions on the existing d3-force package more widely used types, from DefinitelyTyped: https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/d3-force/v2/index.d.ts

This module is an extension of d3-force, so for consistency it would be better to base it on those types, and extend it with the relevant sections, namely adding the 3rd (z) dimension, plus the numDimensions method.

[Aarebecca]

@vasturiano thank you very much for your advice. It was really an oversight on my part.
I've redefined the types based on d3-force, which reduces some of the duplication and ensures consistency.

The screenshot below is a type check performed by ts-server (in strict mode), and everything is fine: