Introduce WP_Labels class

[alpipego]

Classes like WP_Taxonomy and WP_Post_Type return a stdClass object for the $labels property. The way this is done right now, it should be an array not an object.

A solution would be to introduce an abstract WP_Labels class that holds all shared the properties (name, singular_name, search_items, etc.) and concrete classes, e.g. WP_Taxonomy_Labels for type-specific properties (choose_from_most_used, separate_items_with_commas, etc).

see also #48681.