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

Co-authored-by: Quentin Nerden <quentin.nerden@intel.com>

plugins/invoke/README.md

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+# 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

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