Update path settings to "machine" scope to avoid sync settings issues  #548

VScode is currently trialing settings sync in beta. Assuming this makes it through to production it could cause issues for people who store their R executable in different locations on different machines.

According to the documentation you can prevent these paths from being sync'd if you give them a "machine" scope, though I'll be honest and say I don't fully understand what the wider implications of making this change would be.