Missing union type support for Kotlin codegen

```
Execution failed for task ':pkl-playground:genKotlin'.
> Pkl union types are not supported by the Kotlin code generator.
```

---

Bit of a tall ask, I know, but it'd be wonderful if we could map union types into some form of Kotlin structure.

Start by running the pkl-playground:genKotlin task and inspect the Kotlin code generator's handling of Pkl types. The issue names no files or tests and leaves the target Kotlin structure unspecified; done would require an agreed representation for union types and successful Kotlin generation for them.

Written by the indexing model from the issue text.