feat(kn): add plugin for `kn` completion (#8927)

plugins/kn/README.md

+# kn - Knative CLI
+
+This plugin provides autocompletion for [kn](https://knative.dev/docs/install/client/install-kn/) operations.
+
+To use it, add `kn` to the plugins array of your zshrc file:
+
+```zsh
+plugins=(... kn)
+```
+
+## See Also
+
++ [kn/client](https://github.com/knative/client)
+
+## Contributors
+
++ [btannous](https://github.com/btannous) - Plugin Author

plugins/kn/kn.plugin.zsh

+# Autocompletion for kn, the command line interface for knative
+#
+# Author: https://github.com/btannous
+
+if [ $commands[kn] ]; then
+  source <(kn completion zsh)
+  compdef _kn kn 
+fi