Use .NET csproj SDK

[Corniel]

Things have changed since #98 was started. There are some issues left.

1. Proj0043 Generate BOM
2. Proj0215 Icon should be 128x128 not 256x256
3. Proj0050 FP (Proj0050: Enforce code-style in builds has no meaning for non-Roslyn projects dotnet-project-file-analyzers/dotnet-project-file-analyzers#906)
4. Proj1007 FP (F# Only project reports on package.targets to be analyzed using .net.csproj dotnet-project-file-analyzers/dotnet-project-file-analyzers#912)

[Corniel]

Furthermore, I've moved the slnx, and Directory.Build.props to the root, I think this makes cleaner, but it might (at least I expect it to do) break things.