patch 9.1.0120: hard to get visual region using Vim script

patch 9.1.0120: hard to get visual region using Vim script

Problem:  hard to get visual region using Vim script
Solution: Add getregion() Vim script function
          (Shougo Matsushita, Jakub Łuczyński)

closes: #13998
closes: #11579

Co-authored-by: =?UTF-8?q?Jakub=20=C5=81uczy=C5=84ski?= <doubleloop@o2.pl>
Co-authored-by: Shougo Matsushita <Shougo.Matsu@gmail.com>
Signed-off-by: Shougo Matsushita <Shougo.Matsu@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>