feat: add guardian games slot (only to class item)

[Mijago]

I basically add a new category "GuardianGamesClassItem" and show it in the "class item" slot. I can toggle it via an environment flag, so we can easily disable it (or remove it) when we do not need it anymore.

This felt nicer for the end user than adding an extra advanced setting.

for #134

[abea]

The option would fit in with the one for Festival of the Lost, fwiw. Or FOL could switch to this pattern.

[Mijago]

Thank you for bringing this up. I will move fotl to this pattern.

[Mijago]

Adds the Guardian Games slot only to the class item slot:

NOT visible for other slots: