Add TypeScript definition

[BendingBender]

I'm not really happy with the definition. I don't like that many overloads just to support functions with different arity for the fn method.

It would be much easier if onCancel would be passed in as first parameter instead of last:

static fn<ArgumentsType extends unknown[], ReturnType>(
	userFn: (onCancel: OnCancelFunction, ...arguments: ArgumentsType) => PromiseLike<ReturnType>
): (...arguments: ArgumentsType) => PCancelable<ReturnType>;

But I suppose it's not very ergonomic and would break all users' code...