patch 8.2.0111: VAR_SPECIAL is also used for booleans

Problem:    VAR_SPECIAL is also used for booleans.
Solution:   Add VAR_BOOL for better type checking.
diff --git a/src/popupmenu.c b/src/popupmenu.c
index cccde47..0e4f9fe 100644
--- a/src/popupmenu.c
+++ b/src/popupmenu.c
@@ -1076,7 +1076,7 @@
     dict_add_number(dict, "row", pum_row);
     dict_add_number(dict, "col", pum_col);
     dict_add_number(dict, "size", pum_size);
-    dict_add_special(dict, "scrollbar", pum_scrollbar ? VVAL_TRUE : VVAL_FALSE);
+    dict_add_bool(dict, "scrollbar", pum_scrollbar ? VVAL_TRUE : VVAL_FALSE);
 }
 #endif