bug#39512: 28.0.50; Add command isearch-yank-region

[Juri Linkov]

>> I have some advice in that function that lets me call it with no arguments.
>
> Maybe a new function could be added as a wrapper around
> `(funcall region-extract-function)' and with its argument optional.

What do you think about a new function like we added `region-bounds' in
https://lists.gnu.org/archive/html/emacs-devel/2017-01/msg00402.html

(defun region (&optional arg)
  "Return the region content as a string.
For non-contiguous regions return a list of strings."
  (funcall region-extract-function arg))