Implement getTranslation for MixinBlockType and MixinItemType.

`getTranslation` (part of `Translatable`) is currently not implemented for `MixinBlockType` and `MixinItemType`. This implements both by simply creating a static `Translation` object. This'll probably change in the future when something is done with the factories, but works for now.

**Note**: `MixinItemType` calls `StatCollector.translateToLocal` directly at the moment because MC's `Item` only offers the method `getItemStackDisplayName(ItemStack)`. No `ItemStack` is available, and it does not seem right to pass null to it because mods assume it's not null :-).