Add "raw css" functionality

[slafleche]

I've been loving Vanilla Extract so far. The one big feature I haven't found is a way to add "raw" CSS . This is a different feature from from the prefix discussion. Supporting multiple browsers doesn't always get fixed with a Postcss script, you may need different CSS all-together. Sometimes browsers may have experimental styles that you want to try and won't be accepted in the current version of Vanilla Extract. Occasionally I would rather just write out my own css, but still have access to the goodies from Vanilla Extract.

Something similar to: https://typestyle.github.io/#/raw

cssRaw(`
    .${myClass} {
        //... Some styles for browser compatibility
    }
`)