patch 9.2.0609: completion info popup cannot be scrolled with the keyboard

patch 9.2.0609: completion info popup cannot be scrolled with the keyboard

Problem:  The info popup shown beside the insert-mode and command-line
          completion menu can only be scrolled with the mouse wheel, so
          the part below the visible area is unreachable when working
          from the keyboard.
Solution: While the completion menu is shown, scroll the info popup with
          CTRL-SHIFT-Up/Down (one line), CTRL-SHIFT-PageUp/PageDown (one
          page) and CTRL-SHIFT-N/CTRL-SHIFT-P (one line).  The menu stays
          open and the selected item does not change.

related: #20418
fixes:   #20441
closes:  #20444

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>