Support Additional files on GetWorkspace()

[Corniel]

It turned out, that Additional files where not added the ProjectInfo. I created both two tests as a simple implementation. Just let me know if that is sufficient.

[daveaglick]

Thanks for fixing this! And thanks for the review @psfinaki - that makes it easy for me to merge without too much ceremony. I'll be publishing a new minor with just this change momentarily, and then I'll publish a new major that includes the .NET 6 updates from #234 (that way this change will be available to consumers stuck on older .NET versions).

[daveaglick]

The failure here is from the Cake build (the test suite currently builds a handful of other open source projects). I actually just removed that entire test fixture since keeping up with the various edge cases for full third-party codebases was too challenging. We've already got a test suite with different project types and should just continue to add to that as needed.

None of this is your problem, just noting it here for future reference :).