[Bleeding] Add experimental support for entity portal traveling

EntityPortalEvent is called when an entity is about to portal to a
new location. This event is cancellable on top of being able to
change the exit location.

EntityPortalExitEvent is called when exiting the portal, allowing
for adjustment of the exit velocity.

By: EdGruberman <ed@rjump.com>

paper-api/src/main/java/org/bukkit/event/player/PlayerPortalEvent.java

@@ -6,7 +6,7 @@ import org.bukkit.entity.Player;
 import org.bukkit.event.HandlerList;
 
 /**
- * Called when a player completes the portaling process by standing in a portal
+ * Called when a player is about to teleport because it is in contact with a portal
  */
 public class PlayerPortalEvent extends PlayerTeleportEvent {
     private static final HandlerList handlers = new HandlerList();