LibWeb: Serialize StringStyleValue with quotes

In order to access the string's contents, use the new
`StringStyleValue::string_value()` method.

I think I found all the existing places that relied on
`StringStyleValue::to_string()` returning an unquoted string, but it's
hard to know for sure until things break.

Tests/LibWeb/Text/input/css/string-serializes-with-quotes.html

@@ -0,0 +1,8 @@
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        const e = document.createElement("div");
+        e.style.content = '"wheeee"';
+        println(e.style.content);
+    });
+</script>