Implement FirestoreQuery#startAfter

[squirly]

Implements the startAfter query method, simplifies query building, and provides a mechanism for future query limiter and pagination functions.

https://firebase.google.com/docs/firestore/query-data/query-cursors

[dmurvihill]

Hi @squirly, thanks for the contribution. This repo is no longer usable, but I have opened a PR on my new fork and will be publishing a release from there soon after 1 December. Advise to follow that repo if you still rely on firebase-mock.

We are looking for co-maintainers as well.