JS: Publish as NPM package

[JPustkuchen]

Just like fabianwennink/IconCaptcha-PHP#14 for PHP, it would be nice to publish the frontend part as npm package at https://www.npmjs.com/

This is quite simple and would allow to define this as dependency. In the future, splitting these into the different dependency management systems might allow to just require the combination and have the separate libraries updated separately.

For details how to publish to npmjs, see https://docs.npmjs.com/packages-and-modules/contributing-packages-to-the-registry