fix(tailscale): make completion loading async

plugins/tailscale/README.md

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+# tailscale
+
+This plugin provides completion for [tailscale](https://tailscale.com/) (Easy software-defined networks using an implementation of wireguard).
+
+To use it, add `tailscale` to the plugins array in your zshrc file.
+
+```
+plugins=(... tailscale)
+```
+
+**Author:** [@lukeab](https://github.com/lukeab)

plugins/tailscale/tailscale.plugin.zsh

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