Add Labextension directory to wheel

[Universe-sudojam]

Resolves this error message when installing the package via pip when building a docker image of Jupyterlab:

File "/opt/conda/lib/python3.11/site-packages/voila_gridstack/__init__.py", line 18, in <module>
 with open(osp.join(HERE, 'labextension', 'package.json')) as fid:
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: '/opt/conda/lib/python3.11/site-packages/voila_gridstack/labextension/package.json'

[github-actions]

👈 Launch a Binder on branch Universe-sudojam/voila-gridstack/main