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.


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git-svn-id: https://core.svn.wordpress.org/trunk@48130 1a063a9b-81f0-0310-95a4-ce76da25c4cd