Merge the example webpack configs in our repo docs and our bundler integration tests

[owenatgov]

What

Figure out how to include the extra configuration in our webpack example ie: postcss, sass, babel etc into the test webpack config in our bundler integration tests (not merged into main at time of writing, currently only exist in https://github.com/alphagov/govuk-frontend/pull/4961) with a view to remove the webpack example and have it only exist in our bundler integration tests.

Why

When #4961 is merged, we'll have 2 example webpack configs in govuk-frontend and it's more efficient to only have one. We know that webpack has widespread use across government but webpack by default "breaks" govuk-frontend (are these still true?) so the example is theoretically useful. The bundler test integration is intended to test our tree shaking against common bundlers, but it could also act as a reliable example of how to use these bundlers against govuk-frontend.

Who needs to work on this

Developers

Who needs to review this

Developers

Done when

• [ ] We've assessed that the webpack example in the docs is still useful
• [ ] We've integrated the docs example into the bundler example
• [ ] We've assessed if the other bundler examples could include more information as well