Better compatibility with other fishing plugins

Fixes #4428

src/main/java/com/gmail/nossr50/events/McMMOReplaceVanillaTreasureEvent.java

@@ -0,0 +1,41 @@
+package com.gmail.nossr50.events;
+
+import org.bukkit.entity.Item;
+import org.bukkit.event.Event;
+import org.bukkit.event.HandlerList;
+import org.bukkit.inventory.ItemStack;
+import org.jetbrains.annotations.NotNull;
+
+public class McMMOReplaceVanillaTreasureEvent extends Event {
+    private @NotNull ItemStack replacementItemStack;
+    private final @NotNull Item originalItem;
+
+    public McMMOReplaceVanillaTreasureEvent(@NotNull Item originalItem, @NotNull ItemStack replacementItemStack) {
+        this.originalItem = originalItem;
+        this.replacementItemStack = replacementItemStack;
+    }
+
+    /** Rest of file is required boilerplate for custom events **/
+    private static final @NotNull HandlerList handlers = new HandlerList();
+
+    @Override
+    public @NotNull HandlerList getHandlers() {
+        return handlers;
+    }
+
+    public static @NotNull HandlerList getHandlerList() {
+        return handlers;
+    }
+
+    public @NotNull ItemStack getReplacementItemStack() {
+        return replacementItemStack;
+    }
+
+    public void setReplacementItemStack(@NotNull ItemStack replacementItemStack) {
+        this.replacementItemStack = replacementItemStack;
+    }
+
+    public @NotNull Item getOriginalItem() {
+        return originalItem;
+    }
+}