fix: allow remark toc to parse headings with url

lib/remark-toc-headings.js

@@ -1,12 +1,14 @@
 import { visit } from 'unist-util-visit'
 import { slug } from 'github-slugger'
+import { toString } from 'hast-util-to-string'
 
 export default function remarkTocHeadings(options) {
   return (tree) =>
     visit(tree, 'heading', (node, index, parent) => {
+      const textContent = toString(node)
       options.exportRef.push({
-        value: node.children[0].value,
-        url: '#' + slug(node.children[0].value),
+        value: textContent,
+        url: '#' + slug(textContent),
         depth: node.depth,
       })
     })