opt in nested serializers automatically

should nested serializers be automatically added in instead of treated as `any`?

e.g.

```
# no decorator
class Foo(serializers.Serializer):
    field = serializers.CharField()


@ts_interface("nested")
class Bar(serializers.Serializer):
    foo = Foo()
```

generates

```
export interface Bar {
    foo: any;
}
```

at the moment. is this intentional?