Wrap headings in anchor tag linking its slug

Author: callingmedic911

src/markdoc/nodes.mjs

@@ -41,11 +41,9 @@ const nodes = {
       let text = children.filter((child) => typeof child === 'string').join(' ')
       let id = attributes.id ?? slugify(text)
 
-      return new Tag(
-        `h${node.attributes.level}`,
-        { ...attributes, id },
-        children,
-      )
+      return new Tag(`a`, { href: `#${id}`, ...attributes }, [
+        new Tag(`h${node.attributes.level}`, { id }, children),
+      ])
     },
   },
   th: {