Add .artifacts folder to Visual Studio .gitignore

We are planning to add a [new output path format](dotnet/designs#281) to .NET projects which will by default put output in an `.artifacts` folder.  So this PR adds that folder to the Visual Studio .gitignore file.

The first support for this will be in .NET 8 Preview 3.  We will be looking for feedback through the preview process, so it's possible that the folder name or other aspects of the design could change as a result.

Author: dsplaisted

VisualStudio.gitignore

@@ -61,6 +61,7 @@ BenchmarkDotNet.Artifacts/
 project.lock.json
 project.fragment.lock.json
 artifacts/
+.artifacts/
 
 # ASP.NET Scaffolding
 ScaffoldingReadMe.txt