LibWeb: Implement CSSRule and CSSStyleDeclaration serialization

There are a handful of FIXME's here, but this seems generally good.
Note that CSS *values* don't get serialized in a spec-compliant way
since we currently rely on StyleValue::to_string() which is ad-hoc.

Userland/Libraries/LibWeb/CSS/CSSGroupingRule.cpp

@@ -30,4 +30,10 @@ void CSSGroupingRule::delete_rule(size_t)
     TODO();
 }
 
+// https://drafts.csswg.org/cssom/#serialize-a-css-rule
+String CSSGroupingRule::serialized() const
+{
+    TODO();
+}
+
 }