patch 9.2.0825: regexp: submatch in a look-behind is empty with the NFA engine

patch 9.2.0825: regexp: submatch in a look-behind is empty with the NFA engine

Problem:  With the NFA engine a sub-expression inside a variable width
          look-behind, e.g. "\v(.)@<=", is empty for the first match on
          every line except the first one.  The old engine is correct
          (Mukundan)
Solution: The look-behind is retried from the previous line, because the
          width of "." is over-estimated.  While scanning that line the
          start state is added at the end of the line, where it gets the
          position of the line break as its start position, even though
          the match actually starts on the next line.  Use the position
          of the start of the next line in that case (Hirohito Higashi).

fixes:  #20802
closes: #20805

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>