Update to webpack 5 to work on latest Node.js 22

[vincentfretin]

I didn't manage to run that project on an old version of Node.js with nvm,
so I updated webpack from version 4 to 5 so we can run the project on latest Node.js LTS 22.
I removed all babel transforms, none were used actually, also removed the unused zip-loader dependency.
webpack 5 doesn't include nodejs polyfills anymore, so it was easier to rewrite the zip worker, Claude Claude Opus 4.5 replaced unzip-js + Node.js polyfills with fflate - a modern, fast, pure JS zip library that works natively in browsers without polyfills. It's also smaller and faster.

[vincentfretin]

I asked about that particular change Claude did.

Original behavior: If beatFiles[beatmapFilename] was undefined, it would return from the callback and never call postMessage. This meant silent failure - the worker would just hang and never respond.

New behavior: With continue, we skip missing beatmaps but always call postMessage. If data.beats ends up empty, zip-loader properly emits songloaderror.

So the new code has better error handling - it fails gracefully with an error message instead of silently hanging.