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#18213fa6fd41a94
Co-authored-by: Girish Palya <girishji@gmail.com>