Purge CSS implementation for next.css

[Andy-set-studio]

Important if—as outlined in the wiki—the next.css name is changed to main.css, this implementation will need to be updated.

Fixes #6996

Old PR

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[devnook]

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[devnook]

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[matthiasrohmer]

As the approach of including all CSS and purging seems to not be ideal anyway, people are used to working with next.css, the needed change is documented and this PR seems to be mostly used for testing, closing once we decide to touch CSS anyway.