add test for class: directive special characters

test/css/samples/directive-special-character/expected.css

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+.foo\:bar.svelte-xyz{color:red}

test/css/samples/directive-special-character/expected.html

@@ -0,0 +1 @@
+<div class="svelte-xyz foo:bar">Hello world</div>

test/css/samples/directive-special-character/input.svelte

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+<script>
+	const enabled = true;
+</script>
+
+<div class:foo:bar={enabled}>Hello world</div>
+
+<style>
+	.foo\:bar {
+		color: red;
+	}
+</style>