Warn/validate commands with optional arguments before non-optional

As described in #489, a command that declares an optional `@Argument` before a non-optional `@Argument` won't ever allow a single-argument call. It would be nice if ArgumentParser provided a validation warning / error so that authors could move to a different design.

Start by reading issue #489 and the handling of @Argument declarations in ArgumentParser. Determine where positional argument ordering can be validated, then add a warning or error for an optional argument before a non-optional one and verify that authors are directed toward a valid design.

Written by the indexing model from the issue text.