Add API to color Shulker entities

By: Matthew <stteg@hotmail.com>

paper-api/src/main/java/org/bukkit/entity/Shulker.java

@@ -1,3 +1,5 @@
 package org.bukkit.entity;
 
-public interface Shulker extends Golem {}
+import org.bukkit.material.Colorable;
+
+public interface Shulker extends Golem, Colorable {}