patch 9.2.0908: cannot use a {} block in a nested :autocmd

patch 9.2.0908: cannot use a {} block in a nested :autocmd

Problem:  At script level a "{" block is only recognized when it is the
          whole argument of :autocmd or :command, so an :autocmd that is
          the command of another :autocmd cannot use a block.  In a :def
          function a trailing "{" is accepted instead, and there any
          command ending in "{", such as "normal! {", is mistaken for the
          start of a block (lacygoill).
Solution: Locate the block by following the argument of the command,
          descending into a nested :autocmd or :command, and use that
          everywhere a block needs to be recognized (Hirohito Higashi).

fixes:  #20918
closes: #20933

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>