[v2.x] Enable Source Link in nuget packages

[henrygab]

Fixed by PR #67.

Source Link enables anyone building NuGet libraries to provide source-level debugging for their users with almost no effort. See also documentation.

What this means for an end user?

When debugging why loading of a world is returning null, they can step into the function, and get line-by-line, source-level debugging... no matter how they got the substrate DLLs into their project.