src: update MakeCallback() function prototype

src: update MakeCallback() function prototype

Make it possible to invoke MakeCallback() on a v8::Value but only for
the variant that takes a v8::Function as the thing to call.

The const char* and v8::String variants still require a v8::Object
because the function to call is looked up as a property on the receiver,
but that only works when the receiver is an object, not a primitive.