Added playlist repository logic.

Harmonia.Core/Data/PlaylistRepository.cs

@@ -0,0 +1,22 @@
+using Harmonia.Core.Models;
+
+namespace Harmonia.Core.Data;
+
+public class PlaylistRepository : JsonFileRepository<Playlist>
+{
+    protected override string DirectoryName => string.Empty;
+
+    protected override string GetNewFileName()
+    {
+        for (int i = 0; i < 1000; i++)
+        {
+            string shortFileName = $"Playlist{i.ToString().PadLeft(3, '0')}.{Extension}";
+            string filePath = Path.Combine(DirectoryName, shortFileName);
+
+            if (File.Exists(filePath) == false)
+                return shortFileName;
+        }
+
+        throw new Exception("Unable to determine new fileName");
+    }
+}