This PR introduces a helper to register routes for "singleton resources" - resources that are either "one of one" or "zero or one".

Example:

Route::singleton('profile', ProfileController::class);

Many of the options that apply to resource controllers can also be used here, such as only and except:

Route::singleton('profile', ProfileController::class)->except('destroy');

When the resource can be zero or one, a PUT request can be used for both creating and updating the resource because we always know the resource's location. The update name that is traditionally attached to the PUT doesn't make this entirely clear, so we could potentially name it something like upsert instead. Personally, I prefer keeping the existing name rather than introducing a new action name.

Singleton resources can also be nested within a standard resource for cases where a resource is singular in the scope of the parent resource:

Route::singleton('videos.thumbnail', VideoThumbnailController::class);

Route model binding would work for the Video model in the above example, but there is no model binding for singleton resources because the URL does not provide enough information to resolve it. In many cases, there may not even be a dedicated model for the singleton resource. In the above example, the thumbnail could be a separate model, but it could equally just be an attribute on the Video model that warrants its own routes.

For parity with the resource methods, there is also an apiSingleton method that excludes the edit action, and there are singletons and apiSingletons methods for bulk registration.