Switch to snupkg package format and publish symbols to nuget.org

[yousif-bugsnag]

Goal

Currently Appveyor attempts to upload the nupkg build artifacts to nuget.org and the .symbols.nupkg artifacts to the 3rd party symbol server nuget.symbsrc.net, which is no longer maintained. NuGet now supports publishing symbol packages directly to NuGet, using the new snupkg package format.

Design

Updates the build to create NuGet symbol packages using the new .snupkg format rather than the now legacy .symbols.nupkg format, and configures Appveyor publish these to nuget.org rather than the 3rd-party smbsrc.net.

[twometresteve]

LGTM, from what I can tell at least!

[yousif-bugsnag]

After looking at the appveyor build log from the previous commit, I noticed that while the snupkg artifacts were being created they were not being found and uploaded by appveyor.

I've tweaked the wildcard path for the build artifacts so that these are now picked up, and confirmed these are now being uploaded by checking the appveyor build log for this commit.