LibWeb: Add a CustomHasProperty trait to WrapperGenerator

We immediately use this in CSSStyleDeclaration to fix that "background"
in element.style did not return true.
This is the mechanism used in css3test.com for detecting support of
features.

Userland/Libraries/LibWeb/Bindings/CSSStyleDeclarationWrapperCustom.cpp

@@ -11,6 +11,13 @@
 
 namespace Web::Bindings {
 
+bool CSSStyleDeclarationWrapper::internal_has_property(JS::PropertyName const& name) const
+{
+    // FIXME: These should actually use camelCase versions of the property names!
+    auto property_id = CSS::property_id_from_string(name.to_string());
+    return property_id != CSS::PropertyID::Invalid;
+}
+
 JS::Value CSSStyleDeclarationWrapper::internal_get(const JS::PropertyName& name, JS::Value receiver) const
 {
     // FIXME: These should actually use camelCase versions of the property names!

Userland/Libraries/LibWeb/CSS/CSSStyleDeclaration.idl

@@ -1,4 +1,4 @@
-[CustomGet,CustomSet]
+[CustomGet,CustomSet,CustomHasProperty]
 interface CSSStyleDeclaration {
 
     readonly attribute unsigned long length;

Userland/Libraries/LibWeb/CodeGenerators/WrapperGenerator.cpp

@@ -813,6 +813,12 @@ public:
 )~~~");
     }
 
+    if (interface.extended_attributes.contains("CustomHasProperty")) {
+        generator.append(R"~~~(
+    virtual bool internal_has_property(JS::PropertyName const&) const override;
+)~~~");
+    }
+
     if (interface.wrapper_base_class == "Wrapper") {
         generator.append(R"~~~(
     @fully_qualified_name@& impl() { return *m_impl; }