Add "numfmt.d.ts" to package.json as types entry

[rmdort]

Can we get this merged ? Very helpful to have a typescript definition.

[borgar]

Sorry for the delay, I've been considering my options here as well as little time to spare.

The work here looks really good, but:

• I do not actively use TypeScript and don't have a good ability to maintain the types in this format. So I am reluctant to accept this because the type definitions would either go stale if I changed things, or would simply deter me from making changes.

• Secondly, I have been considering an overhaul/flattening of the interface and using JSDoc to generate the documentation (and types!) automatically.

I understand the need for types and would like to add them, but I think it needs to be in a form where I can manage them. So my plan at the moment is to redo the interface and add JSDoc types as I go along, basing them on the work here.

If you are impatient then there is of course nothing stopping you from adding and maintaining types over at DefinitelyTyped while the package does not have them. But I really do intend to address this asap.