patch 8.2.2239: Vim9: concatenating lines with backslash is inconvenient

Problem:    Vim9: concatenating lines with backslash is inconvenient.
Solution:   Support concatenating lines starting with '|', useful for
            :autocmd, :command, etc. (closes #6702)
diff --git a/src/proto/vim9script.pro b/src/proto/vim9script.pro
index c386602..0eced60 100644
--- a/src/proto/vim9script.pro
+++ b/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_comment_start(char_u *p);
 void ex_export(exarg_T *eap);
 void free_imports_and_script_vars(int sid);
 void mark_imports_for_reload(int sid);