Convert markdownlint library to an ECMAScript module, replace markdownlint-micromark with micromark, stop publishing (large) markdownlint-browser.js, see https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c for guidance.

test/rules/first-line.cjs

@@ -0,0 +1,17 @@
+// @ts-check
+
+"use strict";
+
+/** @type {import("../../lib/markdownlint.mjs").Rule} */
+module.exports = {
+  "names": [ "first-line" ],
+  "description": "Rule that reports an error for the first line",
+  "tags": [ "test" ],
+  "parser": "none",
+  "function": function rule(params, onError) {
+    // Unconditionally report an error for line 1
+    onError({
+      "lineNumber": 1
+    });
+  }
+};