* build(contrib): add zsh completion
this is not part of the build system yet, so packager managers are
supposed to install the file by themselves.
bash doesn't seem to provide shell completion, zsh embeds its own
completion that is bundled with vim's.
Instead of copying zsh's completion, this generated one via
https://github.com/RobSis/zsh-completion-generator
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Co-authored-by: Eisuke Kawashima <e.kawaschima+github@gmail.com>
