Check capability before unchecked cast

[Jonathing]

Fixes #30, #31, #33, and ferriarnus/Unidentified-Enchantments#16.

The main way capabilities function are by trusting the capability itself with its own type. The type you choose for the capability is cast to Capability<C> upon registration, but you are nevertheless in control of the entire capability and its usage yourself, as long as you implement the required interfaces at minimum. Therefore, doing reckless unchecked casting on overriding getCapability() is going to cause problems because of the way capabilities work.

Your capabilities look good and work great, but make sure to watch for little nuances like these. There's also a bit of repetitive code between the Quiver and Scabbard capabilities that could be better isolated into a single class/interface file (for example, I made CapabilityProvider and IPersistentCapability in ForageCraft 1.17 for this purpose). Also note the usage of CapabilityManager.get() in order to get active capabilities from the game, rather than the now deleted @CapabilityInject annotation.