Includes: Developed to get new system working with inline includes

Adds logic for locating and splitting text nodes.
Adds specific classes to offload tag/content specific logic.

app/Entities/Tools/PageIncludeContent.php

@@ -0,0 +1,68 @@
+<?php
+
+namespace BookStack\Entities\Tools;
+
+use BookStack\Util\HtmlDocument;
+use DOMNode;
+
+class PageIncludeContent
+{
+    protected static array $topLevelTags = ['table', 'ul', 'ol', 'pre'];
+
+    /**
+     * @var DOMNode[]
+     */
+    protected array $contents = [];
+
+    protected bool $isTopLevel;
+
+    public function __construct(
+        string $html,
+        PageIncludeTag $tag,
+    ) {
+        $this->parseHtml($html, $tag);
+    }
+
+    protected function parseHtml(string $html, PageIncludeTag $tag): void
+    {
+        if (empty($html)) {
+            return;
+        }
+
+        $doc = new HtmlDocument($html);
+
+        $sectionId = $tag->getSectionId();
+        if (!$sectionId) {
+            $this->contents = [...$doc->getBodyChildren()];
+            $this->isTopLevel = true;
+            return;
+        }
+
+        $section = $doc->getElementById($sectionId);
+        if (!$section) {
+            return;
+        }
+
+        $isTopLevel = in_array(strtolower($section->nodeName), static::$topLevelTags);
+        $this->isTopLevel = $isTopLevel;
+        $this->contents = $isTopLevel ? [$section] : [...$section->childNodes];
+    }
+
+    public function isInline(): bool
+    {
+        return !$this->isTopLevel;
+    }
+
+    public function isEmpty(): bool
+    {
+        return empty($this->contents);
+    }
+
+    /**
+     * @return DOMNode[]
+     */
+    public function toDomNodes(): array
+    {
+        return $this->contents;
+    }
+}