feat(cli): add ESM config loader

Since Node.js added support for loading ESM modules synchronously [1], the same
mechanism can be reused, assuming an ESM module's file extension to be `.mjs`.

Implements: #5049

[1] https://nodejs.org/api/modules.html#loading-ecmascript-modules-using-require

Author: ByteB4rb1e

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',
@@ -71,7 +72,7 @@ exports.loadConfig = filepath => {
   try {
     if (ext === '.yml' || ext === '.yaml') {
       config = parsers.yaml(filepath);
-    } else if (ext === '.js' || ext === '.cjs') {
+    } else if (ext === '.js' || ext === '.cjs' || ext === '.mjs') {
       config = parsers.js(filepath);
     } else {
       config = parsers.json(filepath);