feat: toc for blog post

lib/remark-toc-headings.js

@@ -0,0 +1,12 @@
+import visit from 'unist-util-visit'
+
+module.exports = function (options) {
+  return (tree) =>
+    visit(tree, 'heading', (node, index, parent) => {
+      options.exportRef.push({
+        value: node.children[1].value,
+        url: node.children[0].url,
+        depth: node.depth,
+      })
+    })
+}