feat: prevent subclassing of built-in types with runtime error

Add a check in the interpreter's class creation path that rejects attempts to subclass any of the seven core built-in types (Class, Fiber, Fn, List, Map, Range, String). When such a subclass is declared, a runtime error is raised with a message like "Class 'SubName' may not subclass a built-in". Also add corresponding test files for each forbidden subclass scenario.

src/wren_vm.c

@@ -1094,6 +1094,19 @@ static bool runInterpreter(WrenVM* vm)
       {
         // TODO: Handle the superclass not being a class object!
         superclass = AS_CLASS(PEEK());
+        if (superclass == vm->classClass ||
+            superclass == vm->fiberClass ||
+            superclass == vm->fnClass ||
+            superclass == vm->listClass ||  /* This covers also OBJ_CLOSURE */
+            superclass == vm->mapClass ||
+            superclass == vm->rangeClass ||
+            superclass == vm->stringClass )
+        {
+          char message[70 + MAX_VARIABLE_NAME];
+          sprintf(message,
+              "Class '%s' may not subclass a built-in", name->value);
+          RUNTIME_ERROR(AS_STRING(wrenNewString(vm, message, strlen(message))));
+        }
       }
 
       int numFields = READ_BYTE();