Use Rollup for adding a hash to output filenames

Overview

In 🐛 Provide cache buster query string on imports Active , we ensured browser cache busting when there's a new release of XB.
However, that would bust the browser cache even if there aren't any UI changes in that release.

Proposed resolution

In #4 and #9 Bálint and Lee proposed having Vite/Rollup output filenames with a hash in them. That would allow us to do releases with backend-only changes where we don't force re-downloading unchanged JavaScript assets.

See https://rollupjs.org/configuration-options/#output-entryfilenames

User interface changes

None.