feat(nsc): add completion plugin

plugins/nsc/README.md

+# NSC plugin
+
+This plugin adds completion for the [NSC](https://github.com/nats-io/nsc).
+
+To use it, add `nsc` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... nsc)
+```
+
+This plugin does not add any aliases.

plugins/nsc/nsc.plugin.zsh

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