feat: add selector support to require-baseline

[rviscomi]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

Add support for validating the Baseline status of selectors like :has()

What changes did you make? (Give an overview)

• added a new Selector handler that checks the validity of all child nodes
• added tests for pseudo-class and pseudo-element selectors
• updated docs

Related Issues

Fixes #60

Is there anything you'd like reviewers to focus on?

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: rviscomi / name: Rick Viscomi (f1187c8, 00ce917, 6ff2cef, 66524b6)

[rviscomi]

I wasn't sure how to handle selectors used within @supports. It seems like CSSTree always parses them as raw text rather than FeatureFunction as suggested in the docs.

Example CSS:

@supports selector(.example) {}

AST:

{
  "type": "Function",
  "loc": {
    "source": "<unknown>",
    "start": {
      "offset": 10,
      "line": 1,
      "column": 11
    },
    "end": {
      "offset": 28,
      "line": 1,
      "column": 29
    }
  },
  "name": "selector",
  "children": [
    {
      "type": "Raw",
      "loc": {
        "source": "<unknown>",
        "start": {
          "offset": 19,
          "line": 1,
          "column": 20
        },
        "end": {
          "offset": 27,
          "line": 1,
          "column": 28
        }
      },
      "value": ".example"
    }
  ]
}

https://astexplorer.net/#/gist/244e2fb4da940df52bf0f4b94277db44/ff8f2e9e7ca35af89743f13ae98f37a69e501e9d

[nzakas]

Thanks for getting this started. The basics are here, there are a couple of edge cases that need addressing.

[nzakas]

This is looking really good. I think the last step here is to update the docs with some examples that use @supports.

[rviscomi]

Great thanks, added another example to the docs.

[nzakas]

LGTM. Thanks so much for this. Great work.