Rename `Argument`, `Option`, and `Command` to disambiguate from other single-word types

[jonsequitur]

    // Consider adding a disambiguator to this type name, so it doesn't feel like "ArgumentException" is related to this type.
    public abstract class Argument : Symbol, IValueDescriptor, ICompletionSource 

[KathleenDollard]

Use CliArgument, CliOption, and CliCommand