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/App.JsonCodeGen.cs

@@ -0,0 +1,9 @@
+using System.Text.Json.Serialization;
+
+namespace SourceGit
+{
+    [JsonSourceGenerationOptions(WriteIndented = true, IgnoreReadOnlyFields = true, IgnoreReadOnlyProperties = true)]
+    [JsonSerializable(typeof(Models.Version))]
+    [JsonSerializable(typeof(ViewModels.Preference))]
+    internal partial class JsonCodeGen : JsonSerializerContext { }
+}