Added the new item “Nihil Ring”

New item "Nihil Ring" added. Units that have this will have the same effect as the Nihil skill. This item is effective just by holding it.

Some bosses have such a skill fever that you can't fight them unless your unit has Nihil, so I think it was a huge psychological burden for players to decide who to use Nihil M on. I implemented this with the idea of having an item that could be used a little more casually.