[WC] Add support for WooCommerce's new product editor

[rafaelurben]

WooCommerce's new (beta) product editor (see links below) is currently not supported by qTranslate-XT. It would be great if support for (at least) single language edit mode could be added.

The product editor uses Gutenberg’s Block Editor.

I have already tried:

• Using "Single Language edit mode"
• Adding custom classes
• Using https://github.com/picasso/zu-translate

Steps to reproduce:

• Enable “Try new product editor (Beta)” in WooCommerce > Settings > Advanced > Features.
• Edit a multilingual product

Links with information about the new feature:

• Developer documentation: https://developer.woocommerce.com/docs/category/product-editor/
• Roadmap post: https://developer.woocommerce.com/roadmap/block-based-product-creation-experience/
• Public beta announcement: https://woocommerce.com/document/new-product-editor-beta/