Sorting: Reorganised book sort code to its own directory

app/Sorting/BookSortMapItem.php

@@ -0,0 +1,40 @@
+<?php
+
+namespace BookStack\Sorting;
+
+class BookSortMapItem
+{
+    /**
+     * @var int
+     */
+    public $id;
+
+    /**
+     * @var int
+     */
+    public $sort;
+
+    /**
+     * @var ?int
+     */
+    public $parentChapterId;
+
+    /**
+     * @var string
+     */
+    public $type;
+
+    /**
+     * @var int
+     */
+    public $parentBookId;
+
+    public function __construct(int $id, int $sort, ?int $parentChapterId, string $type, int $parentBookId)
+    {
+        $this->id = $id;
+        $this->sort = $sort;
+        $this->parentChapterId = $parentChapterId;
+        $this->type = $type;
+        $this->parentBookId = $parentBookId;
+    }
+}