Stop using module.createRequire (problematic under webpack), incorporate and export resolveModule helper from markdownlint-cli2.

lib/defer-require.cjs

@@ -0,0 +1,16 @@
+// @ts-check
+
+"use strict";
+
+/**
+ * Calls require for markdownit.cjs. Used to synchronously defer loading because module.createRequire is buggy under webpack (https://github.com/webpack/webpack/issues/16724).
+ *
+ * @returns {any} Exported module content.
+ */
+function requireMarkdownItCjs() {
+  return require("./markdownit.cjs");
+}
+
+module.exports = {
+  requireMarkdownItCjs
+};