Is your feature request related to a problem? Please describe.

PostCSS 8 came with a new plugin API. Right now Nuxt.js users can use new PostCSS plugins because Nuxt.js uses PostCSS 7.

Describe the solution you'd like

Update PostCSS from ^7.0.0 to ^8.0.0

Additional context

PostCSS 8 changelog

The only breaking changes important for Nuxt.js is dropping Node.js 6 and 8 support.