Tailwind: Purge CSS-in-JS config

What does this MR do and why?

This extends the tailwind_all_the_way.mjs to accept an --only-used flag. Basically tailwind_all_the_way is called in two scenarios:

1. CI, when building the assets (via yarn run pretailwindcss:build): This now uses --only-used. The css_in_js is reduced to only the classes which are found. This gives us a positive list of things to resolve. The resulting file can be found in the artifacts of the compile-test-assets job.
2. CI, when doing static analysis to figure out we could hypothetically (via lint:tailwind-utils): This still compiles the full set of utilities in order to compare it to the full list of utilities from gitlab-ui.

MR acceptance checklist

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Screenshots or screen recordings

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How to set up and validate locally

In rails console (or via toogle: http://127.0.0.1:3000/rails/features)

Feature.enable(:tailwind_all_the_way)

1. In your GDK, add: export TAILWIND_ALL_THE_WAY=true to env.runit
2. Restart webpack / vite: gdk restart webpack / gdk restart vite
3. Visit any page. The page should look visually alright and a CSS file named assets/tailwind_all_the_way-[hash].css should be loaded