Add support for CSS Production bundling

[thescientist13]

• custom properties? (not didn't pierce the Shadow DOM)
  • parts (generalized theming)
• sharable CSS (e.g. Sass)
• postcss-preset-env instead of cssnext

[thescientist13]

Couple thoughts while working on component-simple-slider

1. Extract CSS into bundles, or leave in Shadow DOM? 🤔
   • What are the ramifications performance wise? Does ShadowDOM scale with a bunch on the page? Better with / or without the use of <template> by lit-html?
2. With styles included via inline <style> tags, we can use :host but unless the CSS is run through a preprocessor, like PostCSS, advanced syntax like nested selectors ( & ) won't be transpiled.

[thescientist13]

This looks fun!
https://github.com/bashmish/lit-css