feat(chezmoi): add completion plugin (#12696)

plugins/chezmoi/README.md

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+# chezmoi Plugin
+
+## Introduction
+
+This `chezmoi` plugin sets up completion for [chezmoi](https://chezmoi.io).
+
+To use it, add `chezmoi` to the plugins array of your zshrc file:
+
+```bash
+plugins=(... chezmoi)
+```

plugins/chezmoi/chezmoi.plugin.zsh

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