patch 9.1.0810: cannot easily adjust the |:find| command

Problem:  cannot easily adjust the |:find| command
Solution: Add support for the 'findexpr' option (Yegappan Lakshmanan)

closes: #15901
closes: #15905

Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/proto/option.pro b/src/proto/option.pro
index db92bf2..fce9199 100644
--- a/src/proto/option.pro
+++ b/src/proto/option.pro
@@ -120,6 +120,7 @@
 char_u *get_option_fullname(int opt_idx);
 opt_did_set_cb_T get_option_did_set_cb(int opt_idx);
 char_u *get_equalprg(void);
+char_u *get_findexpr(void);
 void win_copy_options(win_T *wp_from, win_T *wp_to);
 void after_copy_winopt(win_T *wp);
 void copy_winopt(winopt_T *from, winopt_T *to);