#219 Isolate styling with shadow dom

[robatwilliams]

After #230 and attempt to get it all working, we decided to try an alternative approach which keeps things simpler, and more "on-piste" in terms of build tooling.

Fixes #219

[robatwilliams]

Seemed likely this needed to be removed, but I haven't seen it used - Dan, I guess you will when you try embedding it in the website.

With the style isolation, we are changing our ask of consumers from "include this in your global styles somewhere" to "put this file at /styles.css for us". Which is very rigid.

Perhaps we should allow the styles url to be given as a component attribute - including any path and cache-busting suffixes.

Having the whole bundled styles within the js bundle (so we can import it as a string rather than from a URL path) would eliminate the issue altogether, but then we're back into custom build system/tooling territory.

Why is it necessary to have separate sl-style and package-style? If we could have package-style be always used, we could specify it in styleUrl and simplify things a lot, and then do something else (e.g. style tag) for adaptations the demo page requires.