Add type declaration file. The modules are not accessible after installing `newrl-js` in an external project. <img width="1051" alt="Screenshot 2022-02-05 at 6 16 53 PM" src="https://user-images.githubusercontent.com/6903638/152642761-fc8ec639-e77c-4478-9ecc-195c387ae0ac.png">
Add type declaration file. The modules are not accessible after installing
newrl-jsin an external project.