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/CSSImportRule.h

@@ -36,6 +36,8 @@ public:
 private:
     explicit CSSImportRule(AK::URL);
 
+    virtual String serialized() const override;
+
     AK::URL m_url;
     RefPtr<CSSStyleSheet> m_style_sheet;
 };