Restore SourceLink support

[bart-degreed]

It was introduced in #325, but then removed in #619 for reasons unknown to me.

https://github.com/dotnet/sourcelink/blob/master/docs/README.md

[maurei]

It was removed because it made development with Visual Studio Mac impossible see this issue. Looks like the issue has been resolved and we should be able to pull it back