ExperienceOrb merging/stacking API and fixes

Adds an option for maximum exp value when merging orbs

Adds ExperienceOrbMergeEvent
Fired when the server is about to merge 2 experience orbs
as entities. Plugins can cancel it if they want to ensure experience orbs do not lose important
metadata such as spawn reason, or conditionally move data from source to target.

Fixes an issue where the stacked count was not taking into account
for mending repairs and when merging with spigot's merge-on-spawn
logic

== AT ==
public net.minecraft.world.entity.ExperienceOrb count

Co-authored-by: Aikar <aikar@aikar.co>
Co-authored-by: Jake Potrebic <jake.m.potrebic@gmail.com>

paper-server/src/main/java/org/bukkit/craftbukkit/entity/CraftExperienceOrb.java

@@ -18,6 +18,18 @@ public class CraftExperienceOrb extends CraftEntity implements ExperienceOrb {
         this.getHandle().value = value;
     }
 
+    // Paper start - expose count
+    @Override
+    public int getCount() {
+        return this.getHandle().count;
+    }
+
+    @Override
+    public void setCount(final int count) {
+        this.getHandle().count = count;
+    }
+    // Paper end
+
     // Paper start
     public java.util.UUID getTriggerEntityId() {
         return getHandle().triggerEntityId;