[CommandRegistry] Return self from register(_:) for method chaining

[ikesyo]

registry
    .register(FooCommand())
    .register(BarCommand())
    .register(BazCommand())

[mdiep]

Would it make more sense to accept an array of commands instead?

registry.register(
    FooCommand(),
    BarCommand(),
    BazCommand()
)

[ikesyo]

Sounds good, so how about this?

registry
    .register(FooCommand(), BarCommand(), BazCommand())
    .main(defaultVerb: ...) { ... }