Extract CSS at-rules, move descriptors under at-rules

[tidoust]

This creates an atrules property in CSS extracts that contains at-rules syntax as suggested in #1028. Descriptors associated with an at-rule (and all selectors are associated with an at-rule in practice) now appear under the at-rule definition.

The new atrules property contains a value property when a formal syntax could be extracted from the spec, and a descriptors array that lists descriptors associated with the at-rule (if any).

Breaking change: The former descriptors property that appeared at the root level of the extract no longer exists. The same info can be found under the atrules property.

This update also adjusts a couple of tests and adds two new tests on at-rules extraction.

The code can only extract the formal syntax of an at-rule if it is defined in a <pre class="prod"> block. That is the case in most CSS specs, but some of the definitions are done in <pre> tags without any class for now (e.g. @counter-style and @scroll-timeline). The specs need fixing or the extraction logic needs to also parse mere <pre> tags (but that seems fragile).

As opposed to the structure proposed in #1028, the list of descriptors is not indexed by the descriptor's name.

For instance, the @container at-rule would lead to:

{
  "@container": {
    "value": "@container [ <container-name> ]? <container-condition> { <stylesheet> }",
    "descriptors": [
      {
        "name": "width",
        "for": "@container",
        "value": "<length>",
        "type": "range"
      },
      {
        "name": "height",
        "for": "@container",
        "value": "<length>",
        "type": "range"
      },
      {
        "name": "inline-size",
        "for": "@container",
        "value": "<length>",
        "type": "range"
      },
      {
        "name": "block-size",
        "for": "@container",
        "value": "<length>",
        "type": "range"
      },
      {
        "name": "aspect-ratio",
        "for": "@container",
        "value": "<ratio>",
        "type": "range"
      },
      {
        "name": "orientation",
        "for": "@container",
        "value": "portrait | landscape",
        "type": "discrete"
      }
    ]
  }
}

Creating the pull request as draft as I still need to run additional tests to make sure that the code successfully extracts all known at-rules defined in CSS specs (or that the specs need fixing).

[tidoust]

Marking the PR as ready for review as tests confirm that the code correctly extracts anything that can be extracted (see #1028 (comment)).

[dontcallmedom]

I didn't expect this would land so fast, amazing!