Items model fall back to another ressource pack items model

Currently if you have a ressource pack A and a pack B both modifying the diamond sword it will only apply the cases of the top pack of the list.

I suggest that "fallback":"type":"minecraft:model","model":"minecraft:item/diamond_sword"} would go to the next ressource pack file if any before the vanilla item.

If several same cases are present it will still aplly the usual top /down priority, but it would ensure a better integration of multiple ressources packs.