Make Powershell completion script work in constrained mode (#2196)

Creating CompletionResult objects is not allowed in Powershell constrained mode, so return results as strings if constrained mode is enabled

Store results as PsCustomObjects instead of hashtables. This prevents Sort-Object from trying to convert the hashtable to a object, which is blocked in constrained mode.
PsCustomObjects are created using New-Object to work around https://github.com/PowerShell/PowerShell/issues/20767