Support .mocharc.cjs

This allows for `"type": "module"` in package.json while also using a
mocha config file, which was previously impossible.
See: https://nodejs.org/api/esm.html

Compare to jestjs/jest#9086

lib/cli/config.js

@@ -22,6 +22,7 @@ const findUp = require('find-up');
  */
 exports.CONFIG_FILES = [
   '.mocharc.js',
+  '.mocharc.cjs',
   '.mocharc.yaml',
   '.mocharc.yml',
   '.mocharc.jsonc',
@@ -75,7 +76,7 @@ exports.loadConfig = filepath => {
   try {
     if (ext === '.yml' || ext === '.yaml') {
       config = parsers.yaml(filepath);
-    } else if (ext === '.js') {
+    } else if (ext === '.js' || ext === '.cjs') {
       config = parsers.js(filepath);
     } else {
       config = parsers.json(filepath);