patch 8.2.4360: Vim9: allowing use of "s:" leads to inconsistencies

Problem:    Vim9: allowing use of "s:" leads to inconsistencies.
Solution:   Disallow using "s:" in Vim9 script at the script level.
diff --git a/src/proto/userfunc.pro b/src/proto/userfunc.pro
index 4e30d83..b8d1443 100644
--- a/src/proto/userfunc.pro
+++ b/src/proto/userfunc.pro
@@ -24,6 +24,7 @@
 void save_funccal(funccal_entry_T *entry);
 void restore_funccal(void);
 funccall_T *get_current_funccal(void);
+int at_script_level(void);
 void delete_script_functions(int sid);
 void free_all_functions(void);
 int builtin_function(char_u *name, int len);