Adds the supports-at-rule feature

features/supports-at-rule.yml

@@ -0,0 +1,7 @@
+name: supports-at-rule()
+description: "The `at-rule()` function, when used with `@supports`, checks if a CSS at-rule is supported. For example `@supports at-rule(@starting-style)` checks if the `@starting-style` at-rule is supported."
+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/supports-at-rule.yml.dist

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