refactor: migrate project to .net core 8 to resolve the issue that we can not use CommunityToolkit.Mvvm

BallanceTasEditor/Converters/NullableIntStringConverter.cs

@@ -0,0 +1,19 @@
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using System.Windows.Data;
+
+namespace BallanceTasEditor.Converters {
+    public class NullableIntStringConverter : IValueConverter {
+        public object Convert(object value, Type targetType, object parameter, CultureInfo culture) {
+            throw new NotImplementedException();
+        }
+
+        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture) {
+            throw new NotImplementedException();
+        }
+    }
+}