ui: enable IDE Avalonia designer

Author: mjcheetham

src/shared/Git-Credential-Manager/Program.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Threading;
 using Atlassian.Bitbucket;
+using Avalonia;
 using GitHub;
 using GitLab;
 using Microsoft.AzureRepos;
@@ -101,5 +102,11 @@ private static void AppMain(object o)
                 Dispatcher.MainThread.Shutdown();
             }
         }
+
+        // Required for Avalonia designer
+        static AppBuilder BuildAvaloniaApp() =>
+            AppBuilder.Configure<AvaloniaApp>()
+                .UsePlatformDetect()
+                .LogToTrace();
     }
 }