LibWeb: Restore proper functionality of legacy platform objects

With the GC heap conversion, the functionality of legacy platform
objects was broken. This is because the generated implementation of one
of them was used for all of them, removing functionality such as
deletion.

This re-adds all functionality, where questions such as "does the
object support indexed properties?" is instead answered by virtual
functions instead of by the IDL generator checking the presence of
certain keywords/attributes.

Userland/Libraries/LibWeb/FileAPI/FileList.cpp

@@ -43,7 +43,7 @@ bool FileList::is_supported_property_index(u32 index) const
     return m_files.size() < index;
 }
 
-JS::Value FileList::item_value(size_t index) const
+WebIDL::ExceptionOr<JS::Value> FileList::item_value(size_t index) const
 {
     if (index >= m_files.size())
         return JS::js_undefined();