Reimplement MD024/no-duplicate-heading using micromark tokens.

helpers/micromark.cjs

@@ -345,6 +345,20 @@ function getHeadingStyle(heading) {
   return "atx_closed";
 }
 
+/**
+ * Gets the heading text of a Micromark heading token.
+ *
+ * @param {Token} heading Micromark heading token.
+ * @returns {string} Heading text.
+ */
+function getHeadingText(heading) {
+  const headingTexts = filterByTypes(
+    heading.children,
+    [ "atxHeadingText", "setextHeadingText" ]
+  );
+  return headingTexts[0]?.text.replace(/[\r\n]+/g, " ") || "";
+}
+
 /**
  * HTML tag information.
  *
@@ -460,6 +474,7 @@ module.exports = {
   filterByTypes,
   getHeadingLevel,
   getHeadingStyle,
+  getHeadingText,
   getHtmlTagInfo,
   getMicromarkEvents,
   getTokenParentOfType,