Several fixes and new api for experience merging/stacking (#9242)

patches/server/Expand-PlayerItemMendEvent.patch

@@ -23,7 +23,7 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
              itemstack.setDamageValue(itemstack.getDamageValue() - j);
 -            int k = amount - this.durabilityToXp(j);
 +            int k = amount - event.getDurabilityToXpOperation().applyAsInt(j); // Paper - Expand PlayerItemMendEvent
-             this.value = k; // CraftBukkit - update exp value of orb for PlayerItemMendEvent calls
+             // this.value = k; // CraftBukkit - update exp value of orb for PlayerItemMendEvent calls // Paper - the value field should not be mutated here because it doesn't take "count" into account
 +            // Paper start - Expand PlayerItemMendEvent
 +            if (j == 0 && amount == k) { // if repair amount is 0 and no xp was removed, don't do recursion; treat as cancelled
 +                return k;