vim-patch:9.2.1069: duplicated composing character matching in the NFA engine

Problem:  The composing character matching logic is duplicated between the
          NFA_COMPOSING case and the NFA_COMPOSING branch inside
          NFA_START_COLL in nfa_regmatch().
Solution: Factor the two near-identical paths into a match_composing()
          helper (Julien Voisin).

The two blocks differed only in the composing sub-expression's start state
(t->state->out versus t->state->out->out) and in how the NFA_END_COMPOSING
end state is selected, so those stay at the call sites while the shared
matching body moves into match_composing(). A dead loop advancing a local
"sta" past NFA_END_COMPOSING in the \Z branch and a stale commented-out
condition are dropped in the process.

This was found while looking for optimizations opportunities in the regex
engine(s).

closes: vim/vim#21269

https://github.com/vim/vim/commit/7f83a539d65c48c31d3f0fdf6460ca412a639727

Co-authored-by: Julien Voisin <julien.voisin@dustri.org>

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