Need a list serializer for sprite specifiers

[metalgearsloth]

trafficstars

Only thing that uses it is markings but it isn't getting caught by CI atm.

[ElectroJr]

IMO the right way to fix this is to just add a generic list serializer that can use other serializers (see #2973).

[ElectroJr]

Actually, seeing as its not actually using a custom serializer, it should still be using the SpriteSpecifierSerializer, So the CI should still be checking it?