patch 8.2.4607: sourcing buffer lines may lead to errors for conflicts

Problem:    Sourcing buffer lines may lead to errors for conflicts.
Solution:   Add the ++clear argument. (Yegappan Lakshmanan, closes #9991)

src/proto/vim9script.pro

@@ -2,6 +2,7 @@
 int in_vim9script(void);
 int in_old_script(int max_version);
 int current_script_is_vim9(void);
+void clear_vim9_scriptlocal_vars(int sid);
 void ex_vim9script(exarg_T *eap);
 int not_in_vim9(exarg_T *eap);
 int vim9_bad_comment(char_u *p);