Fix incorrect signature of JSONLoader.parse(...)

The previous signature was incorrect.

* The first argument is an object, not a JSON string
* The second argument is optional
* The return type is known, not `any`

I am using this API at the moment and know that this change is correct for r66 to r68 (the advertised version of these definitions).
May I ask why? It would not be correct to pass a number or boolean here, for example.