Add menus endpoints.

[spacedmonkey]

Description

Add menu endpoints from https://github.com/WP-API/menus-endpoints

How has this been tested?

Screenshots

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[spacedmonkey]

CC // @marekhrabe @draganescu

[draganescu]

@spacedmonkey this is awesome news for the Navigation block :)

Is there a Trac ticket we could tie this PR to?

[spacedmonkey]

@draganescu The original ticket is https://core.trac.wordpress.org/ticket/40878
However, I have been working on the feature plugin, so for history, it is best to look there.

This PR has a lot of code in, but barring some very small tweaks ( to change the name from wp/v2 to __experimental), it is a copy and paste from https://github.com/WP-API/menus-endpoints which as already been code reviewed by @TimothyBJacobs and @kadamwhite . This PR should quick to get merged.

[draganescu]

My bad @spacedmonkey :) I didn't pay attention to which repo this is and I thought it was a patch for core (a PR against the wordpress-develop master) 😁

[paaljoachim]

I do believe that
@getdave @retrofox
might want to know about this PR.

[draganescu]

I have worked with this PR in #18869 and have had no issues. Also considering:

• all endpoints are 'hidden' behind __experiemental,
• the code comes straight from the feature plugin where it has been reviewed extensively by the REST API team

we can merge this and use it for the issues that need it.

[getdave]

I'm really pleased to see this got merged 🎉 🎉 🎉 Great work everyone.