NuGet: GitHub SourceLink

Separate nupkg, snupkg

Add PackageReference

https://www.nuget.org/packages/Microsoft.SourceLink.GitHub/

csproj or Directory.Build.props

```xml
<PropertyGroup>
  <IncludeSymbols>true</IncludeSymbols>
  <SymbolPackageFormat>snupkg</SymbolPackageFormat>
</PropertyGroup>
```