Trinkets slot icons.
Trinkets Updated 4.0 hands the slot JSON `icon` to vanilla `Slot#getNoItemIcon()`, which
`AbstractContainerScreen` draws with `GuiGraphicsExtractor#blitSprite` — i.e. a GUI-atlas sprite id.
So `"icon": "spell_engine:container/slots/<name>"` resolves to `textures/gui/sprites/container/slots/<name>.png`.
Mirrors Trinkets Updated's own icons (`trinkets:container/slots/ring`, ...).
