fix(tailscale): load completion when `tailscale` is an alias (#12726)

This makes it work when Tailscale is installed via App Store:
https://tailscale.com/kb/1080/cli?tab=macos.

Co-authored-by: Marc Cornellà <marc@mcornella.com>

plugins/tailscale/tailscale.plugin.zsh

@@ -1,4 +1,4 @@
-if (( ! $+commands[tailscale] )); then
+if (( ! $+commands[tailscale] && ! $+aliases[tailscale] )); then
   return
 fi
 
@@ -7,7 +7,12 @@ fi
 if [[ ! -f "$ZSH_CACHE_DIR/completions/_tailscale" ]]; then
   typeset -g -A _comps
   autoload -Uz _tailscale
-  _comps[tailscale]=_tailscale
+
+  if (( $+commands[tailscale] )); then
+    _comps[tailscale]=_tailscale
+  elif (( $+aliases[tailscale] )); then
+    _comps[${aliases[tailscale]:t}]=_tailscale
+  fi
 fi
 
 tailscale completion zsh >| "$ZSH_CACHE_DIR/completions/_tailscale" &|