SPIGOT-1644: Add END_GATEWAY as a teleport reason

By: Thinkofdeath <thinkofdeath@spigotmc.org>

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

@@ -60,6 +60,11 @@ public class PlayerTeleportEvent extends PlayerMoveEvent {
          * Entity/Player via the specatator menu
          */
         SPECTATE,
+        /**
+         * Indicates the teleportation was caused by a player entering an End
+         * gateway
+         */
+        END_GATEWAY,
         /**
          * Indicates the teleportation was caused by an event not covered by
          * this enum