Update webpack

[tschaub]

This branch reworks things to work with webpack@5 and the latest rollup plugins.

The examples/webpack/example-builder.js plugin needed to be updated to conform with the new webpack plugin API.

The tasks/serialize-workers.js script needed to be updated to work with the latest babel plugin for rollup.

I've confirmed that examples are properly built and served with npm start, that the production build of the examples with npm run build-examples look good, that the legacy build with npm run build-legacy still works, and that npm run build-package still works.

The examples take about 20 seconds longer to build in production mode compared to building with webpack 4 (building examples in dev mode takes about 5 seconds longer). I think it is a known issue that webpack 5 is slower (webpack/webpack#12102). Maybe we can expect improvements in future releases.

I don't think we have to migrate things right now if we don't want to. But I wanted to see what was involved in updating our plugins.

Closes #12133.

[ahocevar]

I think it makes sense to upgrade. Thanks for the investigation, and feel free to merge.