Added velocity data listener

eco-api/src/main/java/com/willfp/eco/core/Prerequisite.java

@@ -58,6 +58,14 @@ public class Prerequisite {
             "Requires server to be running BungeeCord (or a fork)"
     );
 
+    /**
+     * Requires the server to be running an implementation of Velocity.
+     */
+    public static final Prerequisite HAS_VELOCITY = new Prerequisite(
+            () -> ClassUtils.exists("com.velocitypowered.api.event.player.ServerConnectedEvent"),
+            "Requires server to be running Velocity (or a fork)"
+    );
+
     /**
      * If the necessary prerequisite condition has been met.
      */