feat: Add Badge and Badge demo.

src/Ursa/Converters/BadgeContentOverflowConverter.cs

@@ -0,0 +1,20 @@
+using System.Globalization;
+using Avalonia.Data.Converters;
+
+namespace Ursa.Converters;
+
+public class BadgeContentOverflowConverter: IMultiValueConverter
+{
+    public object? Convert(IList<object?> values, Type targetType, object? parameter, CultureInfo culture)
+    {
+        string overflowMark = parameter is string s ? s : "+";
+        if (double.TryParse(values[0]?.ToString(), out var b) && values[1] is int i and > 0)
+        {
+            if (b > i)
+            {
+                return i + overflowMark;
+            }
+        }
+        return values[0];
+    }
+}