Setup Webpack to compile CSS

What does this MR do and why?

This MR configures Webpack to compile Sass to CSS. It is a WIP but does work. See: #438278 (closed)

It uses sass-loader and mini-css-extract-plugin to take Sass entry files and output them in the expected tree like structure to app/assets/builds

How it works

Sprockets when using the new cssbundling-rails expects files to exist in the app/assets/builds directory. Webpack will use compile_css.mjs helper functions to track entry files and output them to their targets.

What doesn't work

webpack-dev-server does not work. Webpack Dev Server, in my limited understanding, intercepts requests and compiles them on the fly. However I don't believe stylesheet_link_tag/Sprockets is configured to be intercepted. Webpacker introduced a Rack middleware that redirected any /assets requests to Webpack server, instead of being sent to rails and the Sprockets engine

According to cssbundling-rails, my assumption is this works as a file-based watcher, and Sprockets will post-process the CSS outputted in app/assets/builds before serving them in the dev environment.

You develop using this approach by running the bundler in watch mode in a terminal with yarn build:css --watch

• I've left a bunch of TODO comments about some behaviour that still needs to be tested. some node_module imports may not work, some relative path imports don't work (specifically themes/_dark.scss produces nothing)

What Works

• Webpack in static mode works

MR acceptance checklist

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

configure GDK with

vite:
  enabled: false
webpack:
  enabled: true
  static: true
css:
  enabled: true

/cc @leipert