build(@vtmn/svelte): inject CSS to `dist` folder by switching to PostCSS

[Tlahey]

Change the rollup configuration. Switch to postcss to insert the @imported css into the dist folder.

This enhancement doesn't required anymore the @vtmn/css-* as dependencies.

In the dist output with this changes :

[Tlahey]

We need a fix from svelte to merged this PR in the futur. See :

• https://github.com/sveltejs/svelte/pull/6646
• https://github.com/sveltejs/svelte/issues/6584

[github-actions[bot]]

:hatched_chick: Canary published successfully !

[Tlahey]

@lauthieb it's ok for us, we can push it :)

[lauthieb]

Thanks a lot @Tlahey & @AxelPeter for this great work!