I was trying to register a new maintenance mode driver in my app, and noticed a paper cut.

This PR adds a facade for maintenance mode, so a new driver can be registered by doing:

MaintenanceMode::extend('test', fn () => ...);

Instead of having to retrieve the underlying manager class:

$this->app->get(MaintenanceModeManager::class)->extend('test', fn () => ...);

This matches up with elsewhere in Laravel, like Mail::extend(...) and Storage::extend(...).

It also binds maintenance and maintenance.manager strings into the container, so they can be used to retrieve the current maintenance mode driver and manager, like other core dependencies. FQNs still work like before.