Make event handler async, while preserving the order of incoming events.

PkmnLibSharpTests/Battling/BattleTests/BasicBattleTests.cs

@@ -1,5 +1,6 @@
 using System.Collections.Generic;
 using System.Linq;
+using System.Threading.Tasks;
 using NUnit.Framework;
 using PkmnLibSharp.Battling;
 using PkmnLibSharp.Battling.ChoiceTurn;
@@ -94,6 +95,7 @@ namespace PkmnLibSharpTests.Battling.BattleTests
             battle.RegisterEventListener(new BattleEventListener(evt =>
             {
                 evts.Add(evt);
+                return Task.CompletedTask;
             }));
             
             battle.SwitchPokemon(0, 0, p1.GetAtIndex(0));