Enable direct ad-hoc imports

`Can't find variable: IDBIndex` when setting up on Expo

I used the same hierarchy I had working on the web. I am not using performance or analytics, so I think all of the underlying components should work.

But, I believe that since there isn't a way (that I can tell) to import only specific components, I end up with an error as if I were importing performance package.

```
import React from 'react'
import { getAuth, connectAuthEmulator } from 'firebase/auth'
import { getFunctions, connectFunctionsEmulator } from "firebase/functions"
import { getStorage, connectStorageEmulator } from "firebase/storage"
import { getFirestore, connectFirestoreEmulator } from "firebase/firestore";
import { FirebaseAppProvider, useFirebaseApp, AuthProvider, StorageProvider, FirestoreProvider } from 'reactfire';

function FirebaseComponents({ children }) {
const app = useFirebaseApp() // a parent component contains a `FirebaseAppProvider`
const auth = getAuth(app)
const functions = getFunctions(app)
const firestoreInstance = getFirestore(app)
const storage = getStorage(app)

return (



{children}



);
}

function AllFirebaseComponents({ children, fire }) {
return (


{children}


)
}

export default AllFirebaseComponents
```

### Version info
"react": "^17.0.2",
"react-native": "~0.63.4",
"reactfire": "^4.2.0",
"firebase": "^9.0.1",

### Test case

TBU, will try to get one of these

### Steps to reproduce

Start a new expo app, attempt to use reactfire. Get error mentioned above.

### Expected behavior

Be able to import only the components needed, ex:
```
import FirebaseAppProvider from 'reactfire/FirebaseAppProvider';
```

### Actual behavior

I get the error mentioned above which I think comes from the performance stuff

Start by reproducing the IDBIndex error in a new Expo app using the listed React Native, Reactfire, and Firebase versions, then inspect how Reactfire exports its providers. Done means consumers can import only the needed component, such as FirebaseAppProvider, without loading unrelated functionality or triggering the error.

Scritto dal modello di indicizzazione a partire dal testo della issue.