chore: reinstate support for @babel/register for .mjs and .es6 extensions

Author: shellscape

lib/config.js

@@ -13,7 +13,7 @@ const { resolve } = require('path');
 
 const rechoir = require('rechoir');
 
-const esmHandler = {
+const esmRegister = {
   module: 'esm',
   register(hook) {
     // register on .js extension due to https://github.com/joyent/node/blob/v0.12.0/lib/module.js#L353
@@ -25,9 +25,9 @@ const esmHandler = {
   }
 };
 const fileTypes = {
-  '.esm.js': esmHandler,
-  '.es6': esmHandler,
-  '.mjs': esmHandler,
+  '.esm.js': [esmRegister],
+  '.es6': ['@babel/register', esmRegister],
+  '.mjs': ['@babel/register', esmRegister],
   '.babel.js': ['@babel/register', 'babel-register', 'babel-core/register', 'babel/register'],
   '.babel.ts': ['@babel/register'],
   '.ts': [