Lint large test repos in parallel (via worker threads) for shorter run times.

test/markdownlint-test-worker.js

@@ -0,0 +1,16 @@
+// @ts-check
+
+"use strict";
+
+const { parentPort, workerData } = require("node:worker_threads");
+const markdownlint = require("../lib/markdownlint").promises.markdownlint;
+
+// eslint-disable-next-line unicorn/prefer-top-level-await
+markdownlint(workerData).then((lintResults) => {
+  // @ts-ignore
+  parentPort.
+    // eslint-disable-next-line unicorn/require-post-message-target-origin
+    postMessage(lintResults);
+  // eslint-disable-next-line n/no-process-exit
+  process.exit();
+});