vim-patch:9.1.1742: complete: preinsert does not work well with 'autocomplete' (#35692)

Problem:  complete: preinsert does not work well with preinsert
Solution: Make "preinsert" completeopt value work with autocompletion
          (Girish Palya)

This change extends Insert mode autocompletion so that 'preinsert' also
works when 'autocomplete' is enabled.

Try: `:set ac cot=preinsert`

See `:help 'cot'` for more details.

closes: vim/vim#18213

fa6fd41a94

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

test/functional/ui/cursor_spec.lua

@@ -262,7 +262,7 @@ describe('ui/cursor', function()
         m.attr = { background = Screen.colors.DarkGray }
       end
       if m.id_lm then
-        m.id_lm = 76
+        m.id_lm = 77
         m.attr_lm = {}
       end
     end