[v4] Remove unused css variables

[madmoizo]

I'm sure it will be built-in at some point but for now if someone wants to optimize the css output by removing unused css var, here is a little vite plugin I made:
https://github.com/madmoizo/sveltekit-t4-repro/blob/main/.vite/tailwindcss-cleaner.ts

Base tailwind app (no classes used):

With the plugin: