Boat entities use NBT to determine wood type, rather than having separate entity IDs

Currently, wood types of boats are differentiated via a Type NBT tag. This is very inconsistent with all other objects in the game that are made out of different types of wood (including the boat items), which use distinct IDs corresponding to their wood type.

To reproduce:

1. /summon boat ~ ~ ~

{Type:jungle}

• A jungle boat is summoned
  2. /summon jungle_boat ~ ~ ~
• The entity is invalid and nothing is summoned