Add .artifacts folder to Visual Studio .gitignore

We are planning to add a [new output path format](https://github.com/dotnet/designs/pull/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.