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.cpp

@@ -579,4 +579,15 @@ bool ResolvedCSSStyleDeclaration::set_property(PropertyID, StringView)
 {
     return false;
 }
+
+String ResolvedCSSStyleDeclaration::serialized() const
+{
+    // https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-csstext
+    // If the computed flag is set, then return the empty string.
+
+    // NOTE: ResolvedCSSStyleDeclaration is something you would only get from window.getComputedStyle(),
+    //       which returns what the spec calls "resolved style". The "computed flag" is always set here.
+    return String::empty();
+}
+
 }