Add this as a self type (#1708)

pkl-codegen-java/src/main/kotlin/org/pkl/codegen/java/JavaCodeGenerator.kt

@@ -751,6 +751,12 @@ class JavaCodeGenerator(
     when (this) {
       PType.UNKNOWN -> OBJECT.nullableIf(nullable)
       PType.NOTHING -> TypeName.VOID
+      PType.MODULE,
+      PType.THIS ->
+        // TODO: support self types: `class Foo<T extends Foo<T>>`
+        throw JavaCodeGeneratorException(
+          "Pkl `${this}` types are not supported by the Java code generator."
+        )
       is PType.StringLiteral -> STRING.nullableIf(nullable)
       is PType.Class -> {
         // if in doubt, spell it out