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/ResolvedCSSStyleDeclaration.h

@@ -24,6 +24,8 @@ public:
     virtual Optional<StyleProperty> property(PropertyID) const override;
     virtual bool set_property(PropertyID, StringView css_text) override;
 
+    virtual String serialized() const override;
+
 private:
     explicit ResolvedCSSStyleDeclaration(DOM::Element&);