Webpack 5 support

[timneutkens]

Follow-up of #9426, opening this as the overarching issue on what to solve:

• Get rid of buildId and use content hashes for page files
• Simplify on-demand-entries
• Make custom webpack plugins compatible with webpack 5
• Make resolutions to webpack 5 use the webpack 5 implementation initially (to allow canary testing / canary to keep being released without any breaking changes)
• Make React Refresh tests pass
• Make sure there are no deprecation warnings
  • chore(webpack5cache): use compilation.getCache() instead of compilation.cache webpack/terser-webpack-plugin#283
  • https://github.com/webpack-contrib/mini-css-extract-plugin (most warnings come from this plugin)
  • Next.js internal webpack plugins warnings
    • Fix some webpack 5 deprecation warnings #15797
    • Solve large portion of webpack 5 deprecation warnings #15806
• Provide polyfills that were removed in webpack 5 like process and Buffer. We'll likely show a warning when a built-in Node.js module is used where it was not intended, this would ensure backwards compat and significantly less breakage in existing applications.
  • Add process and buffer: Add polyfill for process and Buffer in webpack 5 #15499

[todortotev]

@timneutkens If there is anything you would let a non-maintainer help with, I'd be glad to assist.