Support more CSS at-rules in CssStyle definitions

[NorbertSandor]

It would be useful if CssStyle would support CSS at-rules in general. For example: @container, @starting-style, etc.

[bitspittle]

Thank you for filing this issue! I will review missing at-rules before we launch 1.0.

I may break this up into separate bugs at some point. For example, @container seems like a long-awaited, really nice upgrade to media queries.

We recently added support for CSS layers. Hopefully the work done on that will make adding other at-rules easier to do, although I guess we'll see.