Create a TS interface for VaadinRouterView

[haijian-vaadin]

When implementing client-side navigation callbacks, onBeforeEnter, onBeforeLeave, etc, I want to have autocomplete and type safety, so that I don't make typos.

DoD:
Create a TS interface VaadinRouterView that collects all life cycle callbacks.

A ts view would implement this interface to get the callbacks

class MyView extends HTMLElement implements VaadinRouterView{
  onBeforeEnter(location, commands, router) {
    return location.pathname === '/cancel' ? commands.prevent() : {};
  }
}

[vlukashov]

Looks related to #404. Perhaps, when implementing this issue we need to take into account the use case described in the other issues as well.