provide unpkg.com/demo-selection-js

[tpscrpt]

This PR implements the following:

• Create package.json (npm package) to install tooling
• Integrate scripts to produce browser-compatible minified scripts
• Tell unpkg to deliver minified script when hitting https://unpkg.com/demo-selection-js

Everything should work out of the box, if you want to claim selection-js you should:

• Have npm on your machine
• npm adduser to signing/signup
• change name to selection-js or similar (this will be https://unpkg.com/{name})
• npm publish in the cloned Selection-js repo

Then, you'll be able to include the minified version in your webpages with

<script src="https://unpkg.com/{name}"></script>

And also, you can run npm publish whenever you change selection.js to automatically update the minified script.