Enable support for query count

[Rohitbels]

Count documents with aggregation queries
is now supported https://firebase.google.com/docs/firestore/query-data/aggregation-queries.

can we add support for the same,

useCollectionCount?????

I can work on this

[chrisbianca]

@Rohitbels by all means, if you'd like to work on this then please feel free to submit a PR

[Rohitbels]

@chrisbianca I have made the changes but not sure how to test.

I tried using yarn link for local testing but got some errors.

How do you dev test?