project: reorganize the structure of the project.

* remove dotnet-tool.json because the project does not rely on any dotnet tools.
* remove Directory.Build.props because the solution has only one project.
* move src/SourceGit to src. It's not needed to put all sources into a subfolder of src since there's only one project.

src/Models/Notification.cs

@@ -0,0 +1,13 @@
+namespace SourceGit.Models
+{
+    public class Notification
+    {
+        public bool IsError { get; set; } = false;
+        public string Message { get; set; } = string.Empty;
+    }
+
+    public interface INotificationReceiver
+    {
+        void OnReceiveNotification(string ctx, Notification notice);
+    }
+}