… and passes index to callback, adds ParseStringArrayProperty and uses it for extension parsing

…son array into a vector<T>

This makes ParseNumberArrayProperty, ParseIntegerArrayProperty, and ParseStringArrayProperty just special cases of the base function.
ParseObjectArrayProperty is also a special case which replaces ForEachObjInArray.
ParseArrayProperty also resizes the vector to the json array's size before iteration, preventing any extra allocations. As a side effect, this gives pointer stability during the parsing operation.