WebIDL files that contain only enums currently don't actually output any header files. For example, HTML/NavigationType.idl

WebIDL files that contain enums and interfaces insert the enum definition into the Prototype header. This means that when an interface uses an enum from its IDL file in its public interface, we must include the prototype file in the header. This drastically increases the number of header files that must be included in our translation units.

We should instead try to generate separate header files for each enum. Unfortunately this makes it difficult to know in the build system what all the files are that will be generated, and whether there's any enum-specific cpp files that need to be generated.

Still some design required for this.