Fix stack trace code preview in debug screen

[sebasbit]

Due to a change in the output of the highlight_file function in PHP 8.3, when working with debugging enabled, the code preview is not displayed.

This PR aims to work with both 8.3 and <8.3 output, here some quick test. Before, the function used <br /> :

<code><span style...>
<span>code</span><br /><span>code</span><br /><span>code</span>
</span>
</code>

Now it uses break lines:

<pre><code style...>
  <span>code</span>
  <span>code</span>
  <span>code</span>
</code></pre>

[thePanz]

Tested on a PHP v8.3 setup, and it fixes the issue!

Muchas gracias @sebasbit !