[fix] support @layer (#7514)

* add test

* add layer to apply check

src/compiler/compile/css/Stylesheet.ts

@@ -170,7 +170,7 @@ class Atrule {
 	}
 
 	apply(node: Element) {
-		if (this.node.name === 'media' || this.node.name === 'supports') {
+		if (this.node.name === 'media' || this.node.name === 'supports' || this.node.name === 'layer') {
 			this.children.forEach(child => {
 				child.apply(node);
 			});

test/css/samples/at-layer/expected.css

@@ -0,0 +1 @@
+@layer base, special;@layer special{div.svelte-xyz{color:rebeccapurple}}@layer base{div.svelte-xyz{color:green}}

test/css/samples/at-layer/input.svelte

@@ -0,0 +1,17 @@
+<div>hello</div>
+
+<style>
+	@layer base, special;
+
+	@layer special {
+		div {
+			color: rebeccapurple;
+		}
+	}
+
+	@layer base {
+		div {
+			color: green;
+		}
+	}
+</style>