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/Scripting/ScripterCore.cs

@@ -17,7 +17,7 @@ using Microsoft.SqlServer.Management.SqlParser.Parser;
 using Microsoft.SqlTools.Hosting.Protocol;
 using Microsoft.SqlTools.ServiceLayer.Connection;
 using Microsoft.SqlTools.ServiceLayer.LanguageServices;
-using Microsoft.SqlTools.ServiceLayer.QueryExecution;
+using Microsoft.SqlTools.ServiceLayer.Utility;
 using Microsoft.SqlTools.ServiceLayer.Workspace.Contracts;
 using Microsoft.SqlTools.Utility;
 using ConnectionType = Microsoft.SqlTools.ServiceLayer.Connection.ConnectionType;