Add this as a self type (#1708)

pkl-codegen-kotlin/src/main/kotlin/org/pkl/codegen/kotlin/KotlinCodeGenerator.kt

@@ -649,6 +649,12 @@ class KotlinCodeGenerator(
     when (this) {
       PType.UNKNOWN -> ANY_NULL
       PType.NOTHING -> NOTHING
+      PType.MODULE,
+      PType.THIS ->
+        // TODO: support self types: `class Foo<T extends Foo<T>>`
+        throw KotlinCodeGeneratorException(
+          "Pkl `${this}` types are not supported by the Kotlin code generator."
+        )
       is PType.StringLiteral -> STRING
       is PType.Class -> {
         // if in doubt, spell it out