Added playlist repository logic.

Harmonia.Core/Data/JsonFileRepository.cs

@@ -0,0 +1,26 @@
+using System.Text.Json.Serialization;
+using System.Text.Json;
+
+namespace Harmonia.Core.Data;
+
+public abstract class JsonFileRepository<TObject> : FileRepository<TObject> where TObject : notnull, new()
+{
+    private readonly JsonSerializerOptions _options = new()
+    {
+        WriteIndented = true,
+        IgnoreReadOnlyProperties = true,
+        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull
+    };
+
+    protected override string Extension => "json";
+
+    protected override TObject Deserialize(Stream stream)
+    {
+        return JsonSerializer.Deserialize<TObject>(stream) ?? new();
+    }
+
+    protected override string Serialize(TObject obj)
+    {
+        return JsonSerializer.Serialize(obj, _options);
+    }
+}