patch 9.2.0349: cannot style non-current window separator

patch 9.2.0349: cannot style non-current window separator

Problem:  cannot style non-current window separator
Solution: Add the VertSplitNC highlighting group
          (Hirohito Higashi).

Add VertSplitNC highlight group for vertical separators of non-current
windows, similar to StatusLine/StatusLineNC distinction.  The separator
adjacent to the current window uses VertSplit, others use VertSplitNC.
Default: linked to VertSplit.

At the current window's status line rows, the separator cell is drawn as
a space with the StatusLine highlight so it blends into the status line
without a stray fillchar glyph.

Also:
- :redrawstatus[!] now also redraws vertical separators.
- statusline height changes trigger vsep redraw.

closes: #19951

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>