SPIGOT-4339: Add EntityTransformEvent

Thanks klugemonkey for some contributions in https://hub.spigotmc.org/stash/projects/SPIGOT/repos/bukkit/pull-requests/351/overview

By: nathat890 <nathat890@outlook.com>

paper-api/src/main/java/org/bukkit/event/entity/EntityTransformEvent.java

@@ -0,0 +1,91 @@
+package org.bukkit.event.entity;
+
+import org.bukkit.Warning;
+import org.bukkit.entity.Entity;
+import org.bukkit.event.Cancellable;
+import org.bukkit.event.HandlerList;
+
+/**
+ * Called when an entity is about to be replaced by another entity.
+ *
+ * @deprecated draft API
+ */
+@Deprecated
+@Warning(false)
+public class EntityTransformEvent extends EntityEvent implements Cancellable {
+
+    private static final HandlerList handlers = new HandlerList();
+    private boolean cancelled;
+    private final Entity converted;
+    private final TransformReason transformReason;
+
+    public EntityTransformEvent(Entity original, Entity converted, TransformReason transformReason) {
+        super(original);
+        this.converted = converted;
+        this.transformReason = transformReason;
+    }
+
+    /**
+     * Gets the entity that the original entity was transformed to.
+     *
+     * @return The transformed entity.
+     */
+    public Entity getTransformedEntity() {
+        return converted;
+    }
+
+    /**
+     * Gets the reason for the conversion that has occurred.
+     *
+     * @return The reason for conversion that has occurred.
+     */
+    public TransformReason getTransformReason() {
+        return transformReason;
+    }
+
+    @Override
+    public boolean isCancelled() {
+        return cancelled;
+    }
+
+    @Override
+    public void setCancelled(boolean cancel) {
+        cancelled = cancel;
+    }
+
+    @Override
+    public HandlerList getHandlers() {
+        return handlers;
+    }
+
+    public static HandlerList getHandlerList() {
+        return handlers;
+    }
+
+    public enum TransformReason {
+        /**
+         * When a zombie gets cured and a villager is spawned.
+         */
+        CURED,
+        /**
+         * When a villager gets infected and a zombie villager spawns.
+         */
+        INFECTION,
+        /**
+         * When a entity drowns in water and a new entity spawns.
+         */
+        DROWNED,
+        /**
+         * When a mooshroom (or MUSHROOM_COW) is sheared and a cow spawns.
+         */
+        SHEARED,
+        /**
+         * When lightning strikes a entity.
+         */
+        LIGHTNING,
+        /**
+         * When a slime splits into multiple smaller slimes.
+         */
+        SPLIT
+    }
+}