Breaking changes for Powderhouse

[KathleenDollard]

The Powderhouse update will have significant breaking changes. The existing System.CommandLine will remain available in a preview state to help with the transition.

This list is not complete. This is a working issue so we have a place to stash breaking change notes as we encounter them. Specifically, as we bring tests back on-line, if we have to change them we will enter an issue here. Large-scale API changes, such as the possible unification of the Parser/RootCommand/Configuration will probably not appear here until complete or we have a package available. The primary goal of this issue is to avoid using track of the small changes. Also, specific API usage will not be included here as they will be dynamic.

See #2338 for an explanation of why we are making significant changes. This issue assumes use of the standard pipeline in Powderhouse and direct use of the core parser without the pipeline is considered a new feature.

• Response files are not on by default. A static method in the ResponseSubsystem activates them. The eventual pattern is expected to be adding the subystem to a pipeline, but we will allow their use in direct use of the core parser.

• Remove CommandResult from the public API [Breaking] Remove CommandResult #2367