Set up Source Link for MvvmGen NuGet package to increase debugging experience

[thomasclaudiushuber]

With source link enabled, devs using MvvmGen could also step into the code like EventAggregator while debugging.

Docs: https://learn.microsoft.com/en-us/dotnet/standard/library-guidance/sourcelink

Note: This is not required for the MvvmGen.PureCodeGeneration package, as that one contains no source code, but only a source generator.