Adds the supports-at-rule feature

features/at-rule.yml

@@ -0,0 +1,7 @@
+name: at-rule
+description: "The `at-rule` feature detection is an expansion to feature detection in CSS that allows authors to detect support for at-rules. Example: `@supports at-rule(@starting-style)`"
+spec: 
+  https://www.w3.org/TR/css-conditional-5/
+  https://github.com/w3c/csswg-drafts/pull/12945
+  https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/AtRuleFeatureDetection/explainer.md
+group: css

features/at-rule.yml.dist

@@ -0,0 +1,6 @@
+# Generated from: at-rule.yml
+# Do not edit this file by hand. Edit the source file instead!
+
+status:
+  baseline: false
+  support: {}