feat(fig): add plugin for Fig (#10432)

plugins/fig/fig.plugin.zsh

@@ -0,0 +1,13 @@
+if ! (( $+commands[fig] )); then
+  return
+fi
+
+# If the completion file doesn't exist yet, we need to autoload it and
+# bind it to `fig`. Otherwise, compinit will have already done that
+if [[ ! -f "$ZSH_CACHE_DIR/completions/_fig" ]]; then
+  autoload -Uz _fig
+  typeset -g -A _comps
+  _comps[fig]=_fig
+fi
+
+fig completions zsh >| "$ZSH_CACHE_DIR/completions/_fig" &|