Add a export default to the file that gets required by node

 - Maintains compatibility with direct require('i18next') for plain EcmaScript
 - Adds support for a default import, which fixes the problem with the TypeScript types currently misrepresenting the actual code in a node / require() environment and thus making it usable again

index.js

@@ -1 +1,5 @@
-module.exports = require('./dist/commonjs/index.js').default;
+/* eslint no-var: 0 */
+var main = require('./dist/commonjs/index.js').default;
+
+module.exports = main;
+module.exports.default = main;