postcss8 support for nuxt 2.15+

Since nuxt@2.15 nuxt supports opting-in to use postcss@8 (via nuxt/nuxt.js#8546).

In the meantime, this module allows rapid migration:

Ensures supported nuxt version is used (>= 2.15.3)
Forces to use upgraded dependencies using __NUXT_PREPATHS__
Will be updated accordingly to self-disable when a nuxt version with default postcss8 detected
Use autoprefixer instead of postcss-preset-env

Usage

Install nuxt-postcss8 as devDependency:

yarn add --dev nuxt-postcss8
# or
npm i -D nuxt-postcss8

Add nuxt-postcss8 to buildModules in nuxt.config:

// nuxt.config
export default {
  buildModules: [
    'nuxt-postcss8'
  ]
}

If you have a nuxt module that requires postcss@8, install postcss@8 and nuxt-postcss8 as dependency:

yarn add postcss@8 nuxt-postcss8
# or
npm i postcss@8 nuxt-postcss8

Inside module:

export default async function() {
  await this.addModule(require('nuxt-postcss8'))
}