Show available commands when using --help

[kyoto7250]

This PR adds banner for showing available commands when using --help option.

before

$> steep --help                                                                                                                                                                         
Usage: steep [options]
        --version
        --log-level=LEVEL            Specify log level: debug, info, warn, error, fatal
        --log-output=PATH            Print logs to given path
        --verbose                    Set log level to debug

after

$> steep --help                                                                                                                                                        
Usage: steep [options]

available commands: init, check, validate, annotations, version, project, watch, langserver, stats

Options:
        --version
        --log-level=LEVEL            Specify log level: debug, info, warn, error, fatal
        --log-output=PATH            Print logs to given path
        --verbose                    Set log level to debug

[soutaro]

Thanks! 🎉