vim-patch:9.1.1797: completion: autocompletion can be improved

Problem:  completion: autocompletion can be improved
Solution: Add support for "longest" and "preinsert" in 'autocomplete';
          add preinserted() (Girish Palya)

* Add support for "longest" in 'completeopt' when 'autocomplete'
  is enabled. (Note: the cursor position does not change automatically
  when 'autocomplete' is enabled.)
* Add support for "preinsert" when 'autocomplete' is enabled. Ensure
  "preinsert" works the same with and without 'autocomplete'
* introduce the preinserted() Vim script function, useful for defining
  custom key mappings.

fixes: vim/vim#18314
closes: vim/vim#18387

c05335082a

Co-authored-by: Girish Palya <girishji@gmail.com>

test/functional/editor/completion_spec.lua

@@ -1496,18 +1496,18 @@ describe('completion', function()
 
     -- During delay wait, user can open menu using CTRL_N completion
     feed('<Esc>')
-    command('set completeopt=menuone,preinsert')
+    command('set completeopt=menuone,longest')
     feed('Sf<C-N>')
     screen:expect([[
       foo                                                         |
       foobar                                                      |
       foobarbaz                                                   |
-      f{102:^oo}                                                         |
-      {12:foo            }{1:                                             }|
+      foo^                                                         |
+      {4:foo            }{1:                                             }|
       {4:foobar         }{1:                                             }|
       {4:foobarbaz      }{1:                                             }|
       {1:~                                                           }|*2
-      {5:-- Keyword completion (^N^P) }{6:match 1 of 3}                   |
+      {5:-- Keyword completion (^N^P) }{19:Back at original}               |
     ]])
 
     -- After the menu is open, ^N/^P and Up/Down should not delay