How do I enable developer/debug mode?
On a development site I would suggest uncommenting the following lines in the settings.php file.     emacs sites/default/settings.php

if (file_exists(__DIR__ . '/settings.local.php')) {
  include __DIR__ . '/settings.local.php';
}


and then copying the file example.settings.local.php from /sites folder to /sites/default folder and rename it to settings.local.php

cp sites/example.settings.local.php sites/default/settings.local.php


In addition to adding the following setting

$config['system.logging']['error_level'] = 'verbose';


it also adds a few other settings which will help you in debugging and making development easier. If you don't want any of them in particular, you can always comment them out.

Note : If you think adding a file_exists call to each page will slow down the site, you can always remove it in the production code.