Added tasklist support to markdown exporter

app/Entities/Tools/Markdown/CheckboxConverter.php

@@ -0,0 +1,28 @@
+<?php
+
+namespace BookStack\Entities\Tools\Markdown;
+
+use League\HTMLToMarkdown\Converter\ConverterInterface;
+use League\HTMLToMarkdown\ElementInterface;
+
+class CheckboxConverter implements ConverterInterface
+{
+
+    public function convert(ElementInterface $element): string
+    {
+        if (strtolower($element->getAttribute('type')) === 'checkbox') {
+            $isChecked = $element->getAttribute('checked') === 'checked';
+            return $isChecked ? ' [x] ' : ' [ ] ';
+        }
+
+        return $element->getValue();
+    }
+
+    /**
+     * @return string[]
+     */
+    public function getSupportedTags(): array
+    {
+        return ['input'];
+    }
+}

app/Entities/Tools/Markdown/HtmlToMarkdown.php

@@ -87,6 +87,7 @@ class HtmlToMarkdown
         $environment->addConverter(new CustomParagraphConverter());
         $environment->addConverter(new PreformattedConverter());
         $environment->addConverter(new TextConverter());
+        $environment->addConverter(new CheckboxConverter());
 
         return $environment;
     }