feat: add demo, fix clear button of TimeRangePicker.

demo/Ursa.Demo/ViewModels/DateRangePickerDemoViewModel.cs

@@ -0,0 +1,16 @@
+using System;
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace Ursa.Demo.ViewModels;
+
+public partial class DateRangePickerDemoViewModel: ObservableObject
+{
+    [ObservableProperty] private DateTime? _startDate;
+    [ObservableProperty] private DateTime? _endDate;
+
+    public DateRangePickerDemoViewModel()
+    {
+        StartDate = DateTime.Today;
+        EndDate = DateTime.Today.AddDays(7);
+    }
+}