refactor(export): expose AggregationCursor
refactor(export): expose AggregationCursor

My use case is to be able to do `foo instanceof AggregationCursor` just as one could do with `Cursor` and I don't see any reason why `AggregationCursor` should not be exposed.

mongodb/node-mongodb-native,mongodb/node-mongodb-native,mongodb/node-mongodb-native,mongodb/node-mongodb-native