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/markdownlint-test-parallel.mjs

@@ -0,0 +1,42 @@
+// @ts-check
+
+// eslint-disable-next-line n/no-unsupported-features/node-builtins
+import { availableParallelism } from "node:os";
+import { Worker } from "node:worker_threads";
+import { __filename } from "./esm-helpers.mjs";
+import markdownlint from "../lib/markdownlint.mjs";
+const markdownlintSync = markdownlint.sync;
+
+/**
+ * Lint specified Markdown files (using multiple threads).
+ *
+ * @param {import("../lib/markdownlint.mjs").Options} options Configuration options.
+ * @returns {Promise<import("../lib/markdownlint.mjs").LintResults>} Results object.
+ */
+export function markdownlintParallel(options) {
+  const workerCount = availableParallelism();
+  const files = options.files || [];
+  const chunkSize = Math.ceil(files.length / workerCount);
+  const promises = [];
+  for (let i = 0; i < workerCount; i++) {
+    promises.push(new Promise((resolve, reject) => {
+      const workerData = {
+        ...options,
+        "files": files.slice(i * chunkSize, (i + 1) * chunkSize)
+      };
+      const worker = new Worker(__filename(import.meta).replace(/parallel\.mjs$/, "worker.mjs"), { workerData });
+      worker.on("message", resolve);
+      worker.on("error", reject);
+    }));
+  }
+  return Promise.all(promises).then((workerResults) => {
+    const combinedResults = markdownlintSync(null);
+    for (const workerResult of workerResults) {
+      // eslint-disable-next-line guard-for-in
+      for (const result in workerResult) {
+        combinedResults[result] = workerResult[result];
+      }
+    }
+    return combinedResults;
+  });
+}