Add font library commands #566

Introduced in WordPress 6.5, the Font Library allows users to manage fonts directly in the editor. See https://make.wordpress.org/core/2024/03/14/new-feature-font-library/

There are font collections, which contain a predefined list of font families. A font family contains one or more font faces.

We should be able to follow WP_REST_Font_Collections_Controller, WP_REST_Font_Families_Controller and WP_REST_Font_Faces_Controller for implementing CRUD functionality for these entities. If everything lives under the wp font umbrella we can do:

• wp font collection listget (collections can't be added or deleted)
• wp font family list|get|add|delete
• wp font face list|get|add|delete