Allow alias positional parameters to provide defaults

[Rob Vesse]

Currently we support positional parameters as part of alias expansion. However if the user does not provide sufficient inputs to expand these they remain unresolved and are passed to the parser as-is where they usually result in parse errors. It would be nice if positional parameters were enhanced to allow Bash style defaults i.e. ${1:-default}

This would require enhancing the AliasResolver accordingly