Build / publish new snukg symbol packages

There is new symbol package format called snupkg that is better suited to distributing pdb files for enhanced ability to debug applications.

https://blog.nuget.org/20181116/Improved-debugging-experience-with-the-NuGet-org-symbol-server-and-snupkg.html

More details for wanting to move to the new format is within this issue:
https://github.com/AArnott/Nerdbank.Streams/issues/74