patch 9.2.0453: vertical separator of statusline blend into active statusline

patch 9.2.0453: vertical separator of statusline blend into active statusline

Problem:  Since v9.2.0349, the vertical separator cell at status line
	  rows is drawn as a space with StatusLine highlight, hiding the
	  user's 'fillchars' "vert" or "stl"/"stlnc" character at that
	  cell (after v9.2.0349)
Solution: Drop the status line blend.  At status line rows the separator
	  cell goes back to using the status fillchar when adjacent
	  status lines are connected, or the vsep character otherwise.
	  (Same as before v9.2.0348)

Keep the VertSplitNC highlight group introduced in v9.2.0349.  The
highlight (VertSplit vs VertSplitNC) is selected based on whether the
current window is adjacent to the separator at the row.

Vertical separators are redrawn on current-window changes and on
:redrawstatus[!] so the VertSplit/VertSplitNC highlight is updated
immediately.

fixes:   #20089
related: #19951
closes:  #20167

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>