upgrade to vite and vue3

[rixx]

Progress

• lib build
• dev server
• normal build
• wc build
• make sure build/release still works as expected

Problems

• no .min.js / always minified, but the wc build would be nice to have with both. not urgent, but neat for dev server debugging
• styles are not inlined (or rather, styles from App.js are inlined, but my understanding was that customElement: true would cause all components to have inlined css)
• pretalx-schedule.js now needs to be loaded with type=module because of the export default on the last line, which would break all existing pretalx schedules out there, I think?