Bird nests should be a regular inventory

[djmattyg007]

Right now the bird nest block entity checks specifically to see if there's a vanilla hopper directly below it. It would be better if the bird nest was instead a standard inventory with the forge capabilities system, so that anything can be used to remove items from it.

As an example, I'd like to use Starbuncles from Ars Nouveau to transfer items out of bird nests, but I can't because the nest isn't an inventory.