YITH WooCommerce Frequently Bought Together Premium 1.39.0

* New: Support for WooCommerce 4.2
* Update: Plugin framework
* Update: Italian language
* Fix: Allow html in form additional text

* New: Support for WooCommerce 4.1.0
* New: Support for YITH Proteo theme
* Update: Plugin framework
* Update: Italian language
* Fix: Disable add to cart for not purchasable variations
* Dev: Added total_discount variable to yith-wfbt-form.php template

* New: Support for WooCommerce 4.0.0
* New: Support for WordPress 5.4
* Update: Plugin framework
* Update: Language files