improve browser compatibility

• babel:
  • targets.esmodules: target all browsers supporting es modules.
  • debug: list targeted browsers and applied transforms.
• polyfills:
  • I've tried to use usebuiltins but that add imports to core-js node dependencies and doesn't work in browser. Can we fix that?
  • Use core-js as cdn (45.34 Ko) es6 polyfills.
  • Use dom4 as cdn (5.54 Ko) dom polyfills.
  • I also disable transform-async-to-generator because it break cypress test (I don't know why). this concerns { "ios":"10.3", "safari":"10.1" }.
  • I disable transform-template-literals because template literal used in widgets will not be transform. Basic browser support seems ok.
  • We also need to polyfill customElements, it is not posible to import the polyfill because in this case it apply after customElements.define() use. dev must add the <script> in html.

targeted browsers:

{
  "android": "61",
  "chrome": "61",
  "edge": "16",
  "firefox": "60",
  "ios": "10.3",
  "opera": "48",
  "safari": "10.1",
  "samsung": "8.2"
}