Rework assets

[bobvandevijver]

I would love to see an easy way to not use the jsdelivr files, but something that was build using the local encore/assetmapper installation. Basically like how Symfony UX does it: include a custom package.json in the assets, which defines the npm dependencies and the scripts. The developer can then create a own entrypoint to load the assets.

Can probably be combined with #76.