* Added Cross-sells object and schema for the API Cart response.
* Cross-sells are now WC_Product objects that pass through the visibility filter.
* Removed redundant check.
* Updated function doc comment.
* Cleaned up imports.
* Cross-sells item schema (extension of ProductSchema) was removed, and replaced by ProductSchema itself. Cross-sells are direct product representation, and extending this goes out of scope for the task at hand.
* Unit testing for cart response containing cross-sell products.