Add files for chapter 1

Chapter01/WinUI.SimpleSample/WinUI3.SimpleSample/MainViewModel.cs

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+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace WinUI3.SimpleSample
+{
+    public class MainViewModel : INotifyPropertyChanged
+    {
+        public event PropertyChangedEventHandler PropertyChanged;
+        private string _name;
+        
+        public MainViewModel()
+        {
+            _name = "Bob Jones";
+        }
+
+        public string Name
+        {
+            get { return _name; }
+            set
+            {
+                if (_name == value) return;
+                _name = value;
+                PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Name)));
+            }
+        }
+    }
+}