Support CSS layers

[bitspittle]

If possible, it would be nice to give users a way to resolve rare but frustrating CSS specificity issues.

See also:

• https://developer.mozilla.org/en-US/docs/Web/CSS/@layer
• https://panda-css.com/docs/concepts/cascade-layers

First step would be making sure we can even do this, since at the moment, the stylesheet code is provided by Compose HTML and it might be hard to add layer support to it.