Implement at-rule function for use with @supports

[Bramus]

As per CSS WG Decision [1]: Implement at-rule function for use with @supports

Syntax:

```css
/* Just the at-rule name, which returns whether this is a recognized at-rule name at all. */
@supports(at-rule(@foo)) {
  /* @foo is supported */
}
```

```css
/* An at-rule name accompanied by a descriptor declaration, which returns whether the at-rule is recognized, and the given descriptor successfully parses as part of that at-rule.*/
@supports(at-rule(@foo; desc: val)) {
  /* @foo with the descriptor `desc` and value `val` is supported */
}
```

Note that there's a follow-up issue [2] to extend at-rule() with a third syntax which allows passing in an entire at-rule into at-rule().

```css
/* An entire at-rule, which returns whether or not the at-rule as a whole, when parsed as the first and only content in a fresh stylesheet, is valid or dropped. If the at-rule is valid but drops some of its contents as invalid, such as an unknown descriptor, this will still return true. */
@supports(at-rule(@foo bar {baz: qux})) {
  /* The entire at-rule is supported */
}
```

It also supports 
At the moment, this third syntax is still a WIP.

^[1] https://github.com/w3c/csswg-drafts/issues/2463#issuecomment-1016720310
^[2] https://github.com/w3c/csswg-drafts/issues/6966

[Radar WebKit Bug Importer]

<rdar://problem/87884897>