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.h

@@ -24,6 +24,8 @@ public:
     size_t insert_rule(StringView const& rule, size_t index = 0);
     void delete_rule(size_t index);
 
+    virtual String serialized() const;
+
 protected:
     explicit CSSGroupingRule(NonnullRefPtrVector<CSSRule>&&);