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/testdir/gen_opt_test.vim b/src/testdir/gen_opt_test.vim
index 7a4ffa8..30de286 100644
--- a/src/testdir/gen_opt_test.vim
+++ b/src/testdir/gen_opt_test.vim
@@ -155,7 +155,7 @@
       \		['xxx']],
       \ 'concealcursor': [['', 'n', 'v', 'i', 'c', 'nvic'], ['xxx']],
       \ 'completeopt': [['', 'menu', 'menuone', 'longest', 'preview', 'popup',
-      \		'popuphidden', 'noinsert', 'noselect', 'fuzzy', 'menu,longest'],
+      \		'popuphidden', 'noinsert', 'noselect', 'fuzzy', "preinsert", 'menu,longest'],
       \		['xxx', 'menu,,,longest,']],
       \ 'completeitemalign': [['abbr,kind,menu', 'menu,abbr,kind'],
       \		['', 'xxx', 'abbr', 'abbr,menu', 'abbr,menu,kind,abbr',