Add support for Typescript Custom Mapping

[thebrianchen]

API proposal: internal link

Fixes #1888 via a withConverter() methods on CollectionReference, DocumentReference, and Query that allow developers to use Firestore classes with generics.

Example usage:

class Post {
  constructor(readonly title: string, readonly author: string) {}
  byline(): string {
    return this.title + ', by ' + this.author;
  }
}

const PostConverter = {
  toFirestore(post: Post): DocumentData {
    return {title: post.title, author: post.author};
  },
  fromFirestore(snapshot: QueryDocumentSnapshot, options: SnapshotOptions): Post {
    const data = snapshot.data(options)!;
    return new Post(data.title, data.author);
  }
};

const postSnap = await firebase.firestore().collection('posts').withConverter(PostConverter).doc().get();
const post = postSnap.data();
if (post !== undefined) {
  post.title; // string
  post.byline(); // Should be defined 
  post.someNonExistentProperty; // TS error
}

[mikelehen]

After a final review I found just a few more minor things to nit, but this looks really good to me. Thanks for all the effort that went into it. I think it came out pretty slick. Hopefully users agree. 😄

[thebrianchen]

Also went over the docs with Mark, and updated docs after his review.