What problem are you facing?

Cannot use a code like this:

```typescript
db
.collection(snapshot)
.aggregate([])
.out("mycol")
.toArray()
.then(() => {});
```

Note steps for the pipeline are not included because they are not relevant for the issue.

What driver and relevant dependency versions are you using?

```
node: v14.17.5 LTS
npm: 7.21.0
node-mongodb: 4.1.0
```

Steps to reproduce?

```
cd /tmp/
mkdir test
cd test
npm init -y
npm i mongodb
mkdir src/
echo 'import

{ Db } from "mongodb"; let db: Db; db.collection("coll").aggregate([]).out("mycollection").toArray().then(() => {});' > src/index.ts
npx tsc src/index.ts
```

This is the output:
```
src/index.ts:1:83 - error TS2345: Argument of type 'string' is not assignable to parameter of type 'number'.

1 import { Db }

from "mongodb"; let db: Db; db.collection("coll").aggregate([]).out("mycollection").toArray().then(() => {});
```

This is the related line of code: https://github.com/mongodb/node-mongodb-native/blob/b67af3cd/src/cursor/aggregation_cursor.ts#L132

`out` should be either an string or an object:

https://docs.mongodb.com/manual/reference/operator/aggregation/out/