WooCommerce Store Credit 4.5.6

* Tweak - Added compatibility with WC 4.0.
* Tweak - Tested compatibility with WP 5.4.
* Fix - Refresh the coupon list on the 'My Account' page when coupons are updated.
* Fix - Fixed wrong value for the parameter `$file` when calling the register activation hook.
* Fix - Fixed error 404 when visiting the 'Store Credit' endpoint on the 'My Account' page without flushing the rewrite rules.
* Dev - Added filter to tweak whether it's allowed to create coupons with tax included

* Fix - Fixed fatal error when loading the customizer.

* Feature - Add a note to the customer when sending store credit.
* Feature - Filter coupons by customer or email in the coupon list.
* Tweak - Recovered the setting 'Delete after use'.
* Tweak - Keep the fields' values in the 'Send Store Credit' form on failure.
* Tweak - Added admin notice to enable the coupons.
* Tweak - Use the order's currency to display the Store Credit discount in the edit-order screen.