Support side-effect CSS imports

[andy-haynes]

Add support for importing (plain?) stylesheets, e.g.

import '@radix-ui/themes/styles.css';

I'm assuming would entail a kind of build time CSS side loading to get the desired effect, opening the door to potential collisions at best and malicious UI tampering at worst. However if this behavior is limited to the root container, we could probably support this in a non-problematic way.

[mpeterdev]

possible duplicate of #383

technically different import strategy but likely the same end result

[andy-haynes]

I think this would still be a separate (albeit related) issue because supporting the import in JS would require a very different implementation. But to be honest I'm still not sure what exactly this does in a JS module.