Support arg_require_else_help natively in the derive

[epage]

Right now, users of arg_required_else_help must define their args as being overrideable by the subcommand, even if generally required:

#[derive(Parser)]
struct Cli {
    #[arg(long, required = true)]
    input: Option<PathBuf>,
    #[command(subcommand)]
    subcommand: Option<Subcommand>,
}

In the derive, we could instead express this as

#[derive(Parser)]
enum Cli {
    #[command(these_are_top_level_args)]
    Args {
        #[arg(long)]
        input: PathBuf,
    },
    // ...
}

The main question is what to call these_are_top_level_args attribute.

[epage]

Could we just use flatten and rely on it being a variant-struct to distinguish it from other flatten?

Probably not because someone might want to define those args in a different struct and then the syntax would be ambiguous and we wouldn't know what code to generate.