Introduce CSS module scripts

This change extends the JavaScript module system integration to include CSS module scripts, in addition to JavaScript module scripts. These allow web developers to load CSS into a component definition in a manner that interacts seamlessly with other module script types.

Explainer document: https://github.com/w3c/webcomponents/blob/gh-pages/proposals/css-modules-v1-explainer.md

Closes WICG/webcomponents#759. Part of whatwg#4321.

This change includes the integration for the import assertions proposal (https://github.com/tc39/proposal-import-assertions/), thus closing whatwg#5640 and closing whatwg#5883 by superseding it.