patch 9.1.1797: completion: autocompletion can be improved

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: #18314
closes: #18387

Signed-off-by: Girish Palya <girishji@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>