Serialize/deserialize positional records as lists

Appreciate the quick work on getting records implemented for `json_serializable`.

One request that I have is for records that _only_ contain positional fields be serialized/deserialized as lists so that if I have this JSON list of `double`s:

```json
[0.34, 0.123, 0.542, 0.4143]
```

it would be deserialized into:

```dart
(0.34, 0.123, 0.542, 0.4143)
```

and vice versa.