patch 9.1.1056: Vim doesn't highlight to be inserted text when completing

Problem:  Vim doesn't highlight to be inserted text when completing
Solution: Add support for the "preinsert" 'completeopt' value
          (glepnir)

Support automatically inserting the currently selected candidate word
that does not belong to the latter part of the leader.

fixes: #3433
closes: #16403

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/option.h b/src/option.h
index 11fefc3..df5bf4d 100644
--- a/src/option.h
+++ b/src/option.h
@@ -531,6 +531,7 @@
 #define COT_NOSELECT	    0x080   // FALSE: select & insert, TRUE: noselect
 #define COT_FUZZY	    0x100   // TRUE: fuzzy match enabled
 #define COT_NOSORT	    0x200   // TRUE: fuzzy match without qsort score
+#define COT_PREINSERT	    0x400   // TRUE: preinsert
 #ifdef BACKSLASH_IN_FILENAME
 EXTERN char_u	*p_csl;		// 'completeslash'
 #endif