fix: recognize at-rule() feature queries in use-baseline

[Pixel998]

Prerequisites checklist

• I have read the contributing guidelines.

AI acknowledgment

• I did not use AI to generate this PR.
• (If the above is not checked) I have reviewed the AI-generated content before submitting.

What did you do?

/* eslint css/use-baseline: "error" */

@supports at-rule(@scope) {
	@scope (.card) {
		a {
			color: red;
		}
	}
}

What did you expect to happen?

No lint error, because @scope is guarded by its matching feature query.

What actually happened?

use-baseline reported:

At-rule '@scope' is not a widely available baseline feature.

What is the purpose of this pull request?

This PR makes use-baseline recognize @supports at-rule() feature queries, so at-rules used as progressive enhancements are not incorrectly reported as baseline violations.

What changes did you make? (Give an overview)

• Added at-rule() handling for @supports conditions in use-baseline.
• Documented the supported @supports at-rule() pattern.

Related Issues

Blocked on eslint/csstree#148

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