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/validate-json.cjs

@@ -0,0 +1,34 @@
+// @ts-check
+
+"use strict";
+
+const { parse, printParseErrorCode } = require("jsonc-parser");
+
+/** @type {import("../../lib/markdownlint.mjs").Rule} */
+module.exports = {
+  "names": [ "validate-json" ],
+  "description": "Rule that validates JSON code",
+  "tags": [ "test", "validate", "json" ],
+  "parser": "markdownit",
+  "asynchronous": true,
+  "function": (params, onError) => {
+    const fences = params.parsers.markdownit.tokens
+      .filter((token) => token.type === "fence");
+    for (const fence of fences) {
+      if (/jsonc?/i.test(fence.info)) {
+        const errors = [];
+        parse(fence.content, errors);
+        if (errors.length > 0) {
+          const detail = errors.map(
+            (err) => `${printParseErrorCode(err.error)} (offset ${err.offset}, length ${err.length})`
+          ).join(", ");
+          onError({
+            // @ts-ignore
+            "lineNumber": fence.lineNumber,
+            detail
+          });
+        }
+      }
+    }
+  }
+};