feat(plugins): add completion plugin for `invoke` (#7311)

plugins/invoke/README.md

+# Invoke plugin
+
+This plugin adds completion for [invoke](https://github.com/pyinvoke/invoke).
+
+To use it, add `invoke` to the plugins array in your `~/.zshrc` file:
+
+```zsh
+plugins=(... invoke)
+```
+

plugins/invoke/invoke.plugin.zsh

+# Autocompletion for invoke.
+#
+if [ $commands[invoke] ]; then
+  source <(invoke --print-completion-script=zsh)
+fi