Update DestinationPath logic to handle short / relative paths

[DTW-DanWard]

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.

[DTW-DanWard]

Actually, should not assume SourcePath. Use same standard as PowerShell - relative to the calling path.

[DTW-DanWard]

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/