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/LFSObject.cs

@@ -0,0 +1,8 @@
+namespace SourceGit.Models
+{
+    public class LFSObject
+    {
+        public string Oid { get; set; } = string.Empty;
+        public long Size { get; set; } = 0;
+    }
+}