Support @layer property

Hello, I'm trying to make new @layer property work with JSS (wrap all my JSS inside `@layer test { /*all my jss here */ }`), but the only way I was able to do it is hack methods that output JSS to string and hardcode it there. Any suggestions on how to do it properly?

Start by tracing the JSS methods that output styles to strings and examine how an @layer wrapper could be represented without the reported hardcoded string hack. Done means JSS users can wrap all their JSS in an @layer block through supported behavior rather than modifying generated output manually.

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