[exp] Firestore rxfire function names match Firebase SDK names

In v9 of the Firebase SDK, `firebase/firestore` exports [`doc`](https://firebase.google.com/docs/reference/js/v9/firestore_.md#doc) and [`collection`](https://firebase.google.com/docs/reference/js/v9/firestore_.md#collection) as functions. It may be confusing for developers to have to always remember to be careful with their imports. For example, they'd have to do:

```ts
import { doc } from "firebase/firestore";
import { doc as doc$ } from "rxfire/firestore";

const colorDoc = doc(firestoreInstance, "colors", "blue");
doc$.subscribe(colorDoc, () => {
/* ... */
});
```

Maybe we should change the names of rxfire's `doc` and `collection`? Maybe `docSnapshots` and `querySnapshots`?