[B] Rename Fish to FishHook - Add BUKKIT-3856

### Problem
Fish is a badly named class.

### Justification
Future-proofing against Mojang eventually adding actual fish to the game.
Additionally, all other classes in the entity package are very clear in their names, such as `Pig`, while Fish is actually misleading in its name.

### PR Description
This change creates the FishHook class for current use and future transitioning to.
This is a **non-breaking change**.

https://bukkit.atlassian.net/browse/BUKKIT-3856