Flatten should work with deeply nested lists as well

This can be achieved by setting a flag in the flatten extension or directly flattening the list.
Need some more discussion.
```dart
  final nestedList = [
    [
      1,
      [2, 3]
    ],
    [4, 5, 6]
  ];
  final flattened = nestedList.flatten(); // [1, 2, 3, 4, 5, 6]
```