Add support for ObjectTemplate::SetAccessorProperty

[stroiman]

Finally wrote some tests for this baby :)

There's thing in the interface I'm unsure of. I expose the basic SetAccessorProperty which requires FunctionTemplate instances as input.

But mostly, you don't need this instance. That would only be if you actually use the same function object in multiple places, but that's not the normal usage pattern.

So I also created a helper on top, SetAccessorPropertyCallback, which creates the templates. I'm unsure if this should be in the interface. I find it helpful, but I have no objection to remove it.

Regarding the flags, I haven't researched what they do. I discovered, you shouldn't use ReadOnly. I think the only flag that is applicable for this case is DontEnum, which I believe would prevent the property from being iterated in for . in loops. But it's still listed by Object.getOwnPropertyNames. But I didn't validate this, so I didn't add to the documentation.

[tommie]

// READ_ONLY is an invalid attribute for JS setters/getters.

in src/objects/objects.cc, so I think you're right. Seems this may be about const and module exports.