Throw Exception if the required functions are defined in incorrect order

Signed-off-by: davidarendsen <davidarendsen@hey.com>

src/Functions/RawFunction.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace Arendsen\FluxQueryBuilder\Functions;
+
+class RawFunction extends Base
+{
+    /**
+     * @var string $input
+     */
+    private $input;
+
+    public function __construct(string $input)
+    {
+        $this->input = $input;
+    }
+
+    public function __toString()
+    {
+        return $this->input . ' ';
+    }
+}