chore: add css regression test (#11373)

at rules are now left alone and you can have nested css in them which is scoped correctly. This just adds a test so we don't regress in the future

closes #9267

packages/svelte/tests/css/samples/at-rule-nested-class/_config.js

@@ -0,0 +1,3 @@
+import { test } from '../../test';
+
+export default test({});

packages/svelte/tests/css/samples/at-rule-nested-class/expected.css

@@ -0,0 +1,9 @@
+
+	@starting-style {
+		.card.svelte-xyz{
+			height: 0;
+		}
+	}	
+	.card.svelte-xyz {
+		color: red;
+	}

packages/svelte/tests/css/samples/at-rule-nested-class/input.svelte

@@ -0,0 +1,12 @@
+<div class="card"></div>
+
+<style>
+	@starting-style {
+		.card{
+			height: 0;
+		}
+	}	
+	.card {
+		color: red;
+	}
+</style>