Make Firestore collection and doc getters generic

[btoo]

This enables the ability to define the ReturnType of Firestore's collection and doc getters

e.g.

const mySubcollection = myDocument
  ? myDocument.collection('mySubcollection') as firebase.firestore.CollectionReference<MySubcollectionItemType>
  : undefined

becomes

const mySubcollection = myDocument?.collection<MySubcollectionItemType>('mySubcollection')

[changeset-bot[bot]]

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Latest commit: 4fcc4191cbcc51002b7749303bf4a62cca22e579

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[btoo]

@googlebot I signed it!