WooCommerce Pre-Orders

Both orders that are paid up-front and will be paid later will use the same `wc-pre-ordered` order status, just with different `_wc_pre_orders_when_charged` order meta.

This means that other plugins can never know if the pre-order has a payment processed or not based on the status. For things like order exporters, this means that `$order->is_paid()` fails.

Perhaps adding another status to designate this, then adding that via the `woocommerce_order_is_paid_statuses` would help with third party integrations that need to take action for a paid order, regardless of whether it’s a pre-order or not. This may also be useful down the road for plugins like Memberships if admins wanted to start a membership before a pre-order processed.