[9.x] Allow route group method to be chained

[ockle]

This allows the group method to be chain called when defining routing. An example of where this would be useful:

// In RouteServiceProvider
Route::middleware('web')
    ->domain('foo.example.com')
    ->group(base_path('routes/common.php')
    ->group(base_path('routes/foo.php');

Route::middleware('web')
    ->domain('bar.example.com')
    ->group(base_path('routes/common.php')
    ->group(base_path('routes/bar.php');

[PerryvanderMeer]

@ockle dockblocks should be updated too 🚀

[ockle]

@PerryvanderMeer Updated, thanks 👍