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/option.h b/src/option.h
index ef39031..641514d 100644
--- a/src/option.h
+++ b/src/option.h
@@ -596,6 +596,9 @@
 EXTERN int	p_fic;		// 'fileignorecase'
 EXTERN char_u	*p_ft;		// 'filetype'
 EXTERN char_u	*p_fcs;		// 'fillchar'
+#ifdef FEAT_EVAL
+EXTERN char_u	*p_fexpr;	// 'findexpr'
+#endif
 EXTERN int	p_fixeol;	// 'fixendofline'
 #ifdef FEAT_FOLDING
 EXTERN char_u	*p_fcl;		// 'foldclose'
@@ -1168,6 +1171,7 @@
     , BV_EP
     , BV_ET
     , BV_FENC
+    , BV_FEXPR
     , BV_FP
 #ifdef FEAT_EVAL
     , BV_BEXPR