Add config hook for altering the actually used webpack config

[axten]

What would you like?

Hello!
Since I stumbled across the same problem described #25998 and it took me hours to understand cypress webpack config, I would like to suggest a simple solution for this (beside clarifying docs).

Use case

I wont to import a util function in my cypress test from a node_modules package, that is not transpiled to es5.
Another use case is to load json files from a node_modules package with translations.

Feature request

As many other tools like Cypress provide, it would be nice to have a simple way to modify the actually used webpack config.
API could look like this:

// cypress.config.js
export default defineConfig({
  configureWebpack: (webpackConfig) => {
    return webpackConfig;
  },
});

Very often, just a single change is needed so overriding the whole preprocessor is really cumbersome.

Why is this needed?

Currently, just a little change to webpack config forces the user to completely override the preprocessor and finding a way to replicate the default webpack config, which is not easy (see #25998)

Other

No response