Reexamine the public path cdn

[jasongrout]

Right now, we hardcode the unpkg CDN in the public path of the amd module in the dist/ directory:

https://github.com/jupyter-widgets/widget-cookiecutter/blob/969471848da747d5aa562b8ab00988e3117e4ccf/%7B%7Bcookiecutter.github_project_name%7D%7D/js/webpack.config.js#L69

I think we should:

1. Figure out if we still need to hardcode it with webpack 5's automatic public path code
2. If we still need to hardcode it, change it to jsdelivr to match what we are doing in ipywidgets 8.