feat(symfony-cli): introduce symfony-cli plugin

plugins/symfony-cli/README.md

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+# Symfony CLI plugin
+
+This plugin adds completion for [Symfony CLI](https://symfony.com/download), the developer tool to help you build, run, and manage your Symfony applications directly from your terminal.
+
+To use it, add `symfony-cli` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... symfony-cli)
+```

plugins/symfony-cli/symfony-cli.plugin.zsh

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