Right now linking to a WordPress entity (post, page, category, etc.) is done by inserting a Link block. The user then selects the entity and the URL of the entity is stored in the Link block's url attribute. This is not ideal, though, as an entity's URL may change.

Instead of selecting a Link, we should allow the user to select a Post, Page, Category, etc. The ID of the entity would then be stored instead of the URL. The correct URL would still be rendered on the frontend using the block's render_callback.

This is necessary for the flow described in #22096.