feat(stern): introduce stern plugin

plugins/stern/README.md

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+# Stern plugin
+
+This plugin adds completion for [Stern](https://github.com/stern/stern), the multi pod and container log tailing for Kubernetes.
+
+To use it, add `stern` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... stern)
+```

plugins/stern/stern.plugin.zsh

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