Fix #911 handle perforce files (#400)

- Additional handling of document events with "perforce:" or other SCM strings
- Much of the handling had been added already, but adding in additional validation and ensuring that everywhere `Workspace.GetFile` is called we add a not-null check to avoid null reference errors

src/Microsoft.SqlTools.ServiceLayer/QueryExecution/DataStorage/SaveAsCsvFileStreamFactory.cs

@@ -7,6 +7,7 @@ using System;
 using System.IO;
 using Microsoft.SqlTools.ServiceLayer.QueryExecution.Contracts;
 using Microsoft.SqlTools.ServiceLayer.SqlContext;
+using Microsoft.SqlTools.ServiceLayer.Utility;
 
 namespace Microsoft.SqlTools.ServiceLayer.QueryExecution.DataStorage
 {