Opt out of cache when WooCommerce cart isn't empty

When on a WooCommerce site, we can opt out of cache if the cart isn't empty by checking for the `woocommerce_items_in_cart` cookie.

In the rewrite rules:
```
// This line
RewriteCond %{HTTP:Cookie} !^.*(comment_author_|wordpress_logged_in|wp-postpass_).*$

// Becomes
RewriteCond %{HTTP:Cookie} !^.*(comment_author_|wordpress_logged_in|wp-postpass_|woocommerce_items_in_cart).*$
```

Maybe this should be subject to a checkbox in the options page.

Start by locating where the rewrite rules are generated and compare the existing cookie exclusion with the example in the issue. Check how cache options are exposed in the options page before deciding whether the WooCommerce exclusion should be configurable. Done means the cart cookie can bypass cache, with the checkbox question resolved; the issue names no test or file.

Written by the indexing model from the issue text.