Port the JupyterLab extension to Jupyter Lab 4

[mwouts]

This is a WIP where I try to keep the Jupyter Lab extension within jupytext.

What is probably needed

• Find out how to tell the build tools that the extension is at packages/labextension
• Update the build tools to use hatchling.build as in https://github.com/mwouts/jupyterlab-jupytext/blob/jupyterlab4/pyproject.toml, rather than jupyter_packaging.setupbase.create_cmdclass
• Modernize setup.py and use pyproject.toml instead if possible

[mwouts]

This was achieved by @mahendrapaipuri in #1140 - closing this one!