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>

runtime/doc/options.txt

@@ -1,4 +1,4 @@
-*options.txt*	For Vim version 9.1.  Last change: 2025 Jan 26
+*options.txt*	For Vim version 9.1.  Last change: 2025 Jan 29
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -2168,6 +2168,12 @@ A jump table for the options with a short description can be found at |Q_op|.
 		    scores when "fuzzy" is enabled. Candidates will appear
 		    in their original order.
 
+	   preinsert
+		    Preinsert the portion of the first candidate word that is
+		    not part of the current completion leader and using the
+		    |hl-ComplMatchIns| highlight group. Does not work when
+		    "fuzzy" is also included.
+
 					*'completepopup'* *'cpp'*
 'completepopup' 'cpp'	string (default empty)
 			global

runtime/doc/version9.txt

@@ -41625,7 +41625,9 @@ Changed~
   the "matches" key
 - |v:stacktrace| The stack trace of the exception most recently caught and
   not finished
-- New option value "nosort" for 'completeopt'
+- New option value for 'completeopt':
+	"nosort"	- do not sort completion results
+	"preinsert"	- highlight to be inserted values
 - add |dist#vim9#Launch()| and |dist#vim9#Open()| to the |vim-script-library|
   and decouple it from |netrw|
 - 'termguicolors' is automatically enabled if the terminal supports the RGB