[WIP] Use post_name for query instead of ID

[getdave]

This is not RfR

What?

Amends the REST API to accept a string based slug when querying for a single wp_navigation post.

Why?

See #38291 (comment).

Essentially we need the REST API to be able to find a given wp_navigation by a slug rather than a post ID. This will ultimately make Navigations more portable across different installs.

Follows a similar path to the Template Parts API.

How?

Amends the rest_controller_class for handling wp_navigation to a custom controller Gutenberg_REST_Navigation_Controller. This then overides the get_post method of the parent classes so that it queries by post_name (slug).

It also registers a new GET route which allows for using a string as the ID.

Testing Instructions

• Enable pretty permalinks on your site
• New tab http://localhost:8888/wp-json/wp/v2/navigation/{{POST_NAME_SLUG_OF_YOUR_NAV_POST}}
• See Navigation details.

Screenshots or screencast

[getdave]

Closing in favour of #42809