Mixed: Get underlying type of a mixed property

[sync-by-unito]

Since all numeric types (int, long, double) are represented as a number when read from our SDK, it makes sense to implement a method to get the underlying type of the property value, if your application (such as MongoDB Realm Studio) care about this.

The API could be an instance method on Realm.Object:

realmObject.getType(propertyName);

getType() implies it works for any property (and not just Mixed properties).