feat: enumerate supported feed item types

Author: Harrison Ifeanyichukwu

src/Enums/FeedItemTypes.php

@@ -0,0 +1,13 @@
+<?php
+declare(strict_types = 1);
+
+namespace Forensic\FeedParser\Enums;
+
+class FeedItemTypes extends \SplEnum
+{
+    const __default = self::RSS_FEED_ITEM;
+
+    const RSS_FEED_ITEM = 1;
+    const ATOM_FEED_ITEM = 2;
+    const RDF_FEED_ITEM = 3;
+}