Add Group, Limit, Map, Sort functions

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

src/Function/Group.php

@@ -0,0 +1,32 @@
+<?php
+
+namespace Arendsen\FluxQueryBuilder\Function;
+
+class Group extends Base {
+
+    /**
+     * @var array $columns
+     */
+    private $columns;
+
+    /**
+     * @var string $mode
+     */
+    private $mode;
+
+    public function __construct(array $columns, string $mode = 'by')
+    {
+        $this->columns = $columns;
+        $this->mode = $mode;
+    }
+
+    public function __toString()
+    {
+        $columns = array_map(function($column) {
+			return '"' . $column . '"';
+		}, $this->columns);
+
+        return '|> group(columns: [' . implode(', ', $columns) . '], mode: "' . $this->mode . '") ';
+    }
+
+}