Support URLRouter with include

[jjjkkkjjj]

I re-created a new PR (#2037)

I tried to implement to support URLRouter with include. This will be helpful project structure organized well.

Usage:

In parent's routings.py;

urlpatterns = [
    path('chats/', include('pj.chats.routings'), name='chats'),
]

In child's routings.py;

app_name = 'chats'

urlpatterns = [
    re_path(r"/(?P<room_name>\w+)/chat/$", ChatConsumer.as_asgi()),
]

Also, I've implemented the unittest for it in test_url_router_nesting_by_include in tests/test_routing.py.