* add sourcelink support
* Update sourcelink (2.0.2 -> 2.1.0)
* Add parameter to enable source linking
* tidy up the arguments with an extension method allowing an argument to be conditionally appended
* Add an explicit "test sourcelink" build task and remove the MSBuild one, so we can get some build script output without needing the whole build to be in verbose logging
* run sourcelink test against the NuGet packages
