Make demos work in older browsers

[vlukashov]

As a Vaadin.Router user I want to see the live demos working in all supported browsers. As a Vaadin.Router developer I want to write demos as any other application code (i.e. using modern language features and browser APIs).

DoD:

• it's OK to write non-transpiled code in <vaadin-demo-snippet>s and snippets work in the browsers that require that code to be transpiled first
• the build script for docs is modified to build separate fragments for each demo snippet
• when a snippet is rendered, the source is displayed on the page 'as written' (i.e. non-transpiled), but the HTML sent to the snippet renderer is taken from the build output of that snippet's fragment.
• update all existing demo snippets to have import {Router} from '@vaadin/router'; instead of const {Router} = window.Vaadin;

[vlukashov]

One idea to consider: use StackBlitz to run live demos. It allows writing demos with modern JS and handles all browsers reasonably well. Here is one sample Vaadin.Router demo: https://stackblitz.com/edit/vaadin-router-starter?embed=1&file=index.js. Works in IE11 as well.

The running demo app can be embedded separately: https://vaadin-router-starter.stackblitz.io

The downsides of this approach are:

• need to keep the StackBlitz projects up-to-date with the demos in the @vaadin/router repo (StackBlitz does have an 'import from GitHub' feature that potentially addresses this, but it only works for Angular-based demos. See ~https://github.com/stackblitz/core/issues/564~ https://github.com/stackblitz/core/issues/340 for details)
• the demos source code would not be a part of the @vaadin/router demos page which affects SEO (this can be addressed by including the demos source into the page along with a link to StackBlitz)
• the existing address bar control would not work: -- either embed StackBlitz demos with their own address bar (example) (but that also adds lots of other UI controls, and that makes the screen look very crowded) -- or embed StackBlitz demos chrome-less (example) and add some custom integration code into every demo to make it work with <vaadin-demo-iframe-renderer>.