[9.x] Allow terser singleton bindings

[timacdonald]

It is currently possible to bind singletons to a service provider using the following:

class AppServiceProvider
{
    public $singletons = [
        Abstract::class => Concrete::class,
    ];
}

Often I want to bind a singleton to the container that isn't backed by contract:

class AppServiceProvider
{
    public $singletons = [
        Concrete::class => Concrete::class,
    ];
}

This always trips me up, as I first try:

class AppServiceProvider
{
    public $singletons = [
        Concrete::class,
    ];
}

This PR makes that work. I expect that to work as it is possible within the register function, thus this also creates symmetry between the two methods of registering singletons:

class AppServiceProvider
{
    public function register()
    {
        $this->app->singleton(Concrete::class);
    }
}