Rename namespace Function keyword to Functions to prevent errors

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

tests/Functions/GroupFunctionTest.php

@@ -0,0 +1,18 @@
+<?php
+declare(strict_types=1);
+
+use Arendsen\FluxQueryBuilder\Functions\Group;
+use PHPUnit\Framework\TestCase;
+
+final class GroupFunctionTest extends TestCase {
+
+    public function testSimpleGroup()
+    {
+        $expression = new Group(['foo', 'bar'], 'by');
+
+        $query = '|> group(columns: ["foo", "bar"], mode: "by") ';
+
+        $this->assertEquals($query, $expression->__toString());
+    }
+
+}