Prohibit numbered parameters as method arguments; fix #4892

As of CRuby2.7 it is only warned. `mruby` prohibits explicitly to
implement the future Ruby3 behavior.

mrbgems/mruby-compiler/core/parse.y

@@ -3538,6 +3538,11 @@ f_bad_arg       : tCONSTANT
                       yyerror(p, "formal argument cannot be a class variable");
                       $$ = 0;
                     }
+                | tNUMPARAM
+                    {
+                      yyerror(p, "formal argument cannot be a numbered parameter");
+                      $$ = 0;
+                    }
                 ;
 
 f_norm_arg      : f_bad_arg
@@ -5969,9 +5974,6 @@ parser_yylex(parser_state *p)
             nvars->cdr = nint(-1);
             nvars = nvars->car;
           }
-          if (intn(p->nvars->cdr) < 0) {
-            yywarning(p, "numbered parameter in nested block");
-          }
           pylval.num = n;
           p->lstate = EXPR_END;
           return tNUMPARAM;