Default value for record fields

Since Version 6.7.0 Records are supported.

If I am right, there is currently no possibility to set a default value for a field inside the record.

For example:
```dart
@JsonSerializable()
class MyClass {
final ({
double numberWithDefault,
String someOtherText,
}) myRecord;

@JsonKey(defaultValue: 0.0)
final double anotherNumberWithDefault;

MyClass({
required this.myRecord,
required this.anotherNumberWithDefault,
});

factory MyClass.fromJson(Map json) => _$MyClassFromJson(json);
Map toJson() => _$MyClassToJson(this);
}
```

It would be nice to be able to set `numberWithDefault` to a default value, if the json value is null (just like `anotherNumberWithDefault`).