Switch from webpack to rollup

[mliebelt]

Webpack is really complex, and I am not sure if I use it correctly. For the other (simpler) projects, I use rollup.

Check if rollup has all features to do the packaging of the application in a good-enough way, so that I would have the same technology (again) after the migration. If rollup is not able to do the job, document the reason, so that I would be able (later) to check again.