* 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.
* use PNG instead of SVG for external tool icons and remove dependency `Avalonia.SVG`
* remove unused property `IsVSCodeFound` and `IsFleetFound`
* find VS from registry first
* remove compile warning CA1416
* remove unused enum `OS.Platforms`