support loading ESM configuration files

Author: thescientist13

lib/cli/config.js

@@ -24,6 +24,7 @@ const utils = require('../utils');
 exports.CONFIG_FILES = [
   '.mocharc.cjs',
   '.mocharc.js',
+  '.mocharc.mjs',
   '.mocharc.yaml',
   '.mocharc.yml',
   '.mocharc.jsonc',
@@ -64,13 +65,16 @@ const parsers = (exports.parsers = {
  * @returns {Object} Parsed config object
  */
 exports.loadConfig = filepath => {
+  const packageJson = require(path.resolve(utils.cwd(), 'package.json'));
   let config = {};
   debug('loadConfig: trying to parse config at %s', filepath);
 
   const ext = path.extname(filepath);
   try {
     if (ext === '.yml' || ext === '.yaml') {
       config = parsers.yaml(filepath);
+    } else if ((ext === '.js' && packageJson.type === "module") || ext === '.mjs') {
+      config = parsers.js(filepath).default;
     } else if (ext === '.js' || ext === '.cjs') {
       config = parsers.js(filepath);
     } else {