Dynamically set public path in generated extensions according to page config.
Dynamically set public path in generated extensions according to page config.

This uses the idea from https://github.com/webpack/webpack/issues/10352#issuecomment-675649389 to get the public path set in remoteEntry.js. Eventually, we might be able to specify an init option for code to run when a remoteEntry file is loaded.

Fixes #8827

jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab