Define a code style for our codebase

As I was working on upgrading our codebase to ES6, I started to notice that our code was not very consistent: https://github.com/EclipseFdn/solstice-assets/pull/254

Let use this issue to discuss a code style for JS and (less). I will start the discussion, how about we adopt Airbnb Style guide: https://github.com/airbnb/javascript

We should also end up with a prettier config file to make sure that the whole team shares these settings. https://prettier.io/docs/en/configuration.html