Add typedefs for core/navigation-link block attributes

[getdave]

Let's add typedefs for the core/navigation-link block attributes and share these with the navigation editor. This will make it easier to understand the various fields and what they do and also how these map to nav_menu_item's.

---

Also optional, since we can always follow up later, but a typedef also works well for any enums we need to define:

https://developer.wordpress.org/block-editor/contributors/code/coding-guidelines/#custom-types

Originally posted by @gwwar in #31004 (comment)