patch 8.2.2533: Vim9: cannot use a range with :unlet

Problem:    Vim9: cannot use a range with :unlet.
Solution:   Implement ISN_UNLETRANGE.
diff --git a/src/proto/list.pro b/src/proto/list.pro
index 4ccc897..192981d 100644
--- a/src/proto/list.pro
+++ b/src/proto/list.pro
@@ -20,6 +20,7 @@
 listitem_T *list_find(list_T *l, long n);
 long list_find_nr(list_T *l, long idx, int *errorp);
 char_u *list_find_str(list_T *l, long idx);
+listitem_T *list_find_index(list_T *l, long *idx);
 long list_idx_of_item(list_T *l, listitem_T *item);
 void list_append(list_T *l, listitem_T *item);
 int list_append_tv(list_T *l, typval_T *tv);