Trigger _doing_it_wrong if wp_send_json is used on a REST API request

[rmccue]

We should heavily discourage users from using wp_send_json (& co.) on REST API requests. Two changes I want to make:

1. If _doing_it_wrong is triggered on a REST request, we should add a header ala the _deprecated_function headers.
2. If wp_send_json is called on a REST request, we should trigger a _doing_it_wrong and point people towards WP_REST_Response and WP_Error

[rachelbaker]

Moved into Future Release, needs a patch.

[TimothyBlynJacobs]

I still see this occurring in the wild. Milestoning for 5.5.

Trac ticket: https://core.trac.wordpress.org/ticket/36271

[TimothyBlynJacobs]

@SergeyBiryukov could you take a look at this one if you have a chance?

[whyisjake]

In 48361:

REST API: Trigger _doing_it_wrong() if wp_send_json() is used on a REST API request

In addition to triggering the _doing_it_wrong() logging, also adds a X-WP-DoingItWrong header.

Fixes #36271.

Props rmccue, TimothyBlynJacobs.

[SergeyBiryukov]

In 48367:

REST API: Correct the check for $version argument in rest_handle_doing_it_wrong().

Move WP_REST_Response and WP_Error class names out of the translatable string.

Follow-up to [48327], [48361].

See #36271.

[spacedmonkey]

I would not have used

if ( defined( 'REST_REQUEST' ) && REST_REQUEST ) {

here. I would have used.

if ( wp_is_json_request() ) {

CC @whyisjake @SergeyBiryukov

[SergeyBiryukov]

@TimothyBlynJacobs Could you double-check comment:10?

At a glance, checking REST_REQUEST seems correct to me, as the message is specific to REST API, but I'm not deeply familiar with the history here.

[TimothyBlynJacobs]

The constant comes from #50318.

[TimothyBlynJacobs]

Reclosing this, if we want to adjust the constant check it should be done in that ticket.