Remove Listener namespace

We may reverse this in the future, but for now just going for
consistency between domains (the majority do not use a Listener
namespace)

src/Admin/CheckCustomLessFormat.php

@@ -0,0 +1,43 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Flarum\Admin;
+
+use Flarum\Foundation\ValidationException;
+use Flarum\Settings\Event\Serializing;
+use Illuminate\Contracts\Events\Dispatcher;
+use Less_Exception_Parser;
+use Less_Parser;
+
+class CheckCustomLessFormat
+{
+    public function subscribe(Dispatcher $events)
+    {
+        $events->listen(Serializing::class, [$this, 'check']);
+    }
+
+    public function check(Serializing $event)
+    {
+        if ($event->key === 'custom_less') {
+            $parser = new Less_Parser();
+
+            try {
+                // Check the custom less format before saving
+                // Variables names are not checked, we would have to set them and call getCss() to check them
+                $parser->parse($event->value);
+            } catch (Less_Exception_Parser $e) {
+                throw new ValidationException([
+                    'custom_less' => $e->getMessage(),
+                ]);
+            }
+        }
+    }
+}