BrowserHtmlDebugTrait::htmlOutput() should format JSON

Problem/Motivation

When a functional test outputs the HTML response for a JSONAPI response (or any other JSON), the JSON is all one long string which is hard to read.

It would be nice if this were formatted in a human-readable way.

Steps to reproduce

Proposed resolution

Not sure how to do this.

The $message parameter isn't only JSON, it has a prefix that is text, so we can't do something hacky like json_encode(json_decode(), JSON_PRETTY_PRINT).

I was wondering about adding an HTTP middleware in tests to do this https://ourcodeworld.com/articles/read/1120/how-to-pretty-print-a-jsonre... for JSON responses, but JSONAPI module for one isn't using Symfony JSON responses AFAICT.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet