Target netcoreapp 3.1

[jozefchmelar]

I also created an issue #6 and I thought I could give it a try too

I followed official instructions for migrations https://docs.microsoft.com/en-us/dotnet/desktop-wpf/migration/convert-project-from-net-framework

Project was 100% portable to .net core so I used the migration tool

Build works on .net core app and .net framework