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

plugins/fig/README.md

+# Fig plugin
+
+This plugin sets up completion for [Fig](https://fig.io/).
+
+To use it, add `fig` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... fig)
+```

plugins/fig/fig.plugin.zsh

+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" &|