Add UserValidatorByMatrixUserName.

app/Models/Validation/Validators/UserValidatorByMatrixUserName.php

@@ -0,0 +1,17 @@
+<?php
+
+namespace App\Models\Validation\Validators;
+
+use App\Models\Validation\Validators\NextValidatorByName;
+use App\Models\User;
+
+class UserValidatorByMatrixUserName extends NextValidatorByName
+{
+    public function isCurrentValid(): bool
+    {
+        $count = User::where('matrix_username', $this->getName())->count();
+
+        return $count != 0;
+    }
+}
+