REST API: Duplicate widget types in /wp/v2/widget-types

[noisysocks]

The /wp/v2/widget-types endpoint is returning duplicate widget types which causes the issue described in ​https://github.com/WordPress/gutenberg/issues/32019.

Using ​https://httpie.io:

$ http -a admin:password GET http://wp-git-build.test/wp-json/wp/v2/widget-types | jq '.[].id'
"pages"
"calendar"
"archives"
"media_audio"
"media_image"
"media_gallery"
"media_video"
"meta"
"search"
"search"
"text"
"categories"
"recent-posts"
"recent-comments"
"rss"
"tag_cloud"
"nav_menu"
"custom_html"
"block"
"block"
"block"
"block"
"block"
"block"
"block"
"block"
"block"
"block"
"block"
"block"
"block"
"block"

Modifies the /wp/v2/widget-types endpoint to de-duplicate widget types by their id. Duplicates appear because WP_Widget::_register will add one entry to $wp_registered_widgets per widget instance.

Trac ticket: https://core.trac.wordpress.org/ticket/53305

[SergeyBiryukov]

#53302 was marked as a duplicate.

[spacedmonkey]

@noisysocks
I also had a ticket for this at #53302

Also related #53303

Now that the widgets array has key, we could ksort to make sure widget types are in an alphabetical order.

[TimothyBlynJacobs]

In 51049:

REST API: Remove duplicates in the widget types endpoint.

Props noisysocks, spacedmonkey, imath, isabel_brison.
Fixes #53305.