patch 8.2.2620: Vim9: Using #{ for a dictionary gives strange errors

Problem:    Vim9: Using #{ for a dictionary gives strange errors.
Solution:   Give an error when using #{ for a comment after a command.

src/proto/vim9script.pro

@@ -2,6 +2,7 @@
 int in_vim9script(void);
 void ex_vim9script(exarg_T *eap);
 int not_in_vim9(exarg_T *eap);
+int vim9_bad_comment(char_u *p);
 int vim9_comment_start(char_u *p);
 void ex_export(exarg_T *eap);
 void free_imports_and_script_vars(int sid);