Use constructable stylesheets for element styling

[trusktr]

This is a more performant way to share a common styling across instances of a custom element. Not all browsers support it yet, and the API may change a little before the spec settles.

Explainer: https://developers.google.com/web/updates/2019/02/constructable-stylesheets

Polyfill: https://github.com/calebdwilliams/construct-style-sheets

[trusktr]

Safari supports this out of the box now, time to implement this!

[trusktr]

Related: when we implement this,

• inject element's styles at the end of the ShadowRoot children #12

will not apply while styles are adopted (behaves similar to appending to end of ShadowRoot).