feat(poetry): add plugin to provide completion for Poetry (#10595)

plugins/poetry/README.md

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+# Poetry Plugin
+
+This plugin automatically installs [Poetry](https://python-poetry.org/)'s completions for you, and keeps them up to date as your Poetry version changes.
+
+To use it, add `poetry` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... poetry)
+```

plugins/poetry/poetry.plugin.zsh

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+# Return immediately if poetry is not found
+if (( ! $+commands[poetry] )); then
+  return
+fi
+
+# If the completion file doesn't exist yet, we need to autoload it and
+# bind it to `poetry`. Otherwise, compinit will have already done that.
+if [[ ! -f "$ZSH_CACHE_DIR/completions/_poetry" ]]; then
+  typeset -g -A _comps
+  autoload -Uz _poetry
+  _comps[gh]=_poetry
+fi
+
+poetry completions zsh >| "$ZSH_CACHE_DIR/completions/_poetry" &|
+
+unset comp_file