Make the dependency on `jupyter_server` optional

[jtpio]

It looks like the cookiecutter current requires jupyter_server:

extension-cookiecutter-ts/{{cookiecutter.python_name}}/setup.py

Line 53 in ae2a726

"jupyter_server>=1.6,<2"

Probably this is only useful when the lab extension also has a server extension. So we could guard this with has_server_extension like in other places.