add Webcomponent example

[glennsl]

This might be a bit offensive to the Barcelonians, but it's based on the example here: https://github.com/glennsl/leafy-example

Maybe we could find a simple Geojson polygon of Barcelona or Catalonia to use here instead?

[jchavarri]

updated example in https://ml-in-barcelona.github.io/jsoo-react/.