Update DestinationPath logic to handle short / relative paths
If a user does not specify a full path for DestinationPath and that file doesn't already exist (i.e. the Resolve-Path call doesn't work 100% correctly) then assume the user wants the DestinationPath relative to the SourcePath.
Probably something like Join-Path SourcePath DestinationPath but need to handle situation if user has adding ..-type path as prefix.
Actually, should not assume SourcePath. Use same standard as PowerShell - relative to the calling path.
Should probably update path processing per Keith's blog: https://rkeithhill.wordpress.com/2016/02/17/creating-a-powershell-command-that-process-paths-using-visual-studio-code/