Support more @ rules

[WorldSEnder]

Supporting more at rules mainly divides into two (new) categories:

Global at rules that introduce a new name:

• @keyframes: @keyframes <identifier> should work similar to the way a new class name is generated, but generate the name used to refer to the animation.
• @counter-style: @counter-style <identifier> similar as above, but generate the name referring to the counter.

At rules that are always global and not associated to a name, offering conditional styling:

• @font-face: Mount font styles. Pretty self explanatory.
• @page: Not supported in all browsers, but sometimes useful - I used it once in the past for a report sheet that could be printed. Can only modify a subset of css attributes of the document.
• @font-feature-values: Pretty fringe feature. I've never seen it used, but def belongs to this category.

The parser and the ast should be mainly reusable, but emitting the style probably needs to be changed to allow for replacing fragments in at rules. Out of the above keyframes and font-face would be my two priorities.

[simbleau]

Add @-webkit-keyframes to the mix.

[futursolo]

I am not sure why we need to support this since all browser versions that support WebAssembly support @keyframes without vendor prefixing.

[simbleau]

You're right - I learned this yesterday and forgot to remove my comment here. +1

[phatgolo]

Don't forget:

• @container should work exactly like @media works today. I've added a PR for it.