Add index for folder export with extension provided

[sharky98]

Just updated and the file extension work like a charm! But, the folder are still missing. Here a quick pull request that should do the trick.

When using an extension, the folder export must explicitly call the index.js file inside the folder.

// export * from "./folder"; // Wrong folder export
export * from "./folder/index.js"; // Good folder export
export * from "./file.js"; // Good file export