Problem: customlist completion cannot supply pum metadata
Solution: Allow each item returned by a customlist function to be
either a string or a Dict with keys "word", "abbr", "kind",
"menu" and "info" (Yasuhiro Matsumoto).
closes: vim/vim#201005c700152ae
Co-authored-by: Yasuhiro Matsumoto <mattn.jp@gmail.com>