Missing return type for `toArray()`

Currently, `Yallist.toArray()` returns a type of `Array<any>`. However, since `Yallist<T>` is initialized with a specific type `T`, could we instead return `Array<T>` from `Yallist.toArray()`?

---

Perhaps

```ts
toArray() {
    // ...
}
```

could be updated to something like:

```ts
toArray(): Array<T> {
    // ...
}
```
