Added playlist logic.

Harmonia.Core/Events/PlaylistUpdatedEventArgs.cs

@@ -0,0 +1,12 @@
+using Harmonia.Core.Models;
+
+namespace Harmonia.Core.Events;
+
+public class PlaylistUpdatedEventArgs : EventArgs
+{
+    public required PlaylistUpdateAction Action { get; init; }
+    public required int Index { get; init; }
+    public int? NewIndex { get; init; } = -1;
+    public required int Count { get; init; }
+    public required PlaylistSong[] Songs { get; init; } = [];
+}