Add Group, Limit, Map, Sort functions

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

tests/Function/MapFunctionTest.php

@@ -0,0 +1,18 @@
+<?php
+declare(strict_types=1);
+
+use Arendsen\FluxQueryBuilder\Function\Map;
+use PHPUnit\Framework\TestCase;
+
+final class MapFunctionTest extends TestCase {
+
+    public function testSimpleMap()
+    {
+        $expression = new Map('r with name: r.user');
+
+        $query = '|> map(fn: (r) => ({ r with name: r.user })) ';
+
+        $this->assertEquals($query, $expression->__toString());
+    }
+
+}