Kotlin plugin does not create __typename property in data class

**Describe the bug**
Kotlin types are not generated with a `__typename` property which breaks resolving union types.

**To Reproduce**
Steps to reproduce the behavior:

* Generate types using Kotlin plugin
* I tried adding `nonOptionalTypename: true` but that had no effect

https://codesandbox.io/s/quizzical-star-wkmfk?file=/codegen.yml

Start by reproducing the Kotlin plugin generation described in codegen.yml, using the linked CodeSandbox as the example. Check the generated Kotlin data classes and the effect of nonOptionalTypename: true. Done means generated types include a __typename property and union types resolve correctly.

Written by the indexing model from the issue text.