With the exception of highlight styles, the styles in annotator.css are
now only used by UI elements contained within shadow roots. As a result,
we can stop loading annotator.css as a stylesheet into the host page,
preventing these styles from affecting the host page.

 - Stop loading annotator.css into the host page as a stylesheet.
   Instead add it as a preload link instead, ready for use by shadow
   roots.

 - Move highlights styles into a separate bundle and load that in the
   host page.

Fixes https://github.com/hypothesis/client/issues/2979