bugfix: exempt the :root selector from prefix

- the change in #1705 introduced a small bug for users who were relying on assigning global CSS variables via the :root selector
- this change adds a small exemption to avoid prefixing :root with the random prefix that svelte adds

src/compiler/compile/css/Selector.ts

@@ -62,7 +62,9 @@ export default class Selector {
 			while (i--) {
 				const selector = block.selectors[i];
 				if (selector.type === 'PseudoElementSelector' || selector.type === 'PseudoClassSelector') {
-					if (i === 0) code.prependRight(selector.start, attr);
+					if (selector.name !== 'root') {
+						if (i === 0) code.prependRight(selector.start, attr);
+					}
 					continue;
 				}
 

test/css/samples/css-vars/expected.css

@@ -1 +1 @@
-div.svelte-xyz{--test:10}
+:root{--root-test:20}div.svelte-xyz{--test:10}

test/css/samples/css-vars/input.svelte

@@ -1,7 +1,10 @@
 <div></div>
 
 <style>
+	:root {
+		--root-test: 20;
+	}
 	div {
 		--test: 10;
 	}
-</style>
+</style>