playground: fix bug when printing recursive polyvariant types without concrete declarations (#6532)

Author: zth

jscomp/jsoo/jsoo_playground_main.ml

@@ -304,6 +304,7 @@ let rescript_parse ~filename src =
 module Printer = struct
   let printExpr typ =
     Printtyp.reset_names();
+    Printtyp.reset_and_mark_loops typ;
     Res_doc.toString
       ~width:60 (Res_outcome_printer.printOutTypeDoc (Printtyp.tree_of_typexp false typ))