Add .artifacts folder to Visual Studio .gitignore

[dsplaisted]

We are planning to add a new output path format 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.

[github-actions[bot]]

This PR is stale because there have been no updates in 90 days. It will close after 180 days of inactivity. Leave a comment if you want to keep it open :smile:

[cremor]

Please merge this.