Use GitHub Actions for CI + use dotnet CLI for packaging

[rikkit]

This PR switches to GitHub Actions for CI instead of AppVeyor, and reimplements the packaging step using dotnet pack and props in csproj vs using a NuGet.exe and props in nuspec.

TODO

• [x] Figure out versioning
  • use VersionPrefix prop in csproj
• [x] dotnet build
• [x] dotnet test
• [x] dotnet pack
• [x] dotnet nuget push
• [x] Set protected branches on GitHub settings
• [x] Remove AppVeyor config