Add a .vsconfig file

Visual Studio 2019 allows for a new mechanism to communicate requirements for the build: .vsconfig files. They contain information about which workloads and features of Visual Studio must be installed for a given project. You can tread more about them in ths this blog post.

It would be nice to have such a file in the ML.NET repository to make it easier for people to check out & build the code.

we do have .editorconfig file but we should also look into this.