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).
@basarat, interesting. I didn't know or expect that. Seems surprising. Anyway, the PR's been updated as requested, and I learned something :)