dev: PostCSS v8 upgrade, "postcss-cssnext" deprecation

[danielweck]

postcss-loader already supports v8:

https://github.com/webpack-contrib/postcss-loader/blob/master/CHANGELOG.md

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postcss-mixins is ready (but Thorium is currently using the older version to avoid a breaking change):

https://github.com/postcss/postcss-mixins/blob/master/CHANGELOG.md

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postcss-cssnext is deprecated so Thorium should replace this dependency:

https://www.npmjs.com/package/postcss-cssnext

---

postcss-import is not ready yet:

postcss/postcss-import#427

[danielweck]

postcss-preset-env (the replacement for postcss-cssnext) is not ready yet:

csstools/postcss-preset-env#191

Migration notes (we must pay attention to "nesting", I think):

csstools/postcss-preset-env#32

[danielweck]

postcss-import ^13.0.0 has been released:
https://github.com/postcss/postcss-import/blob/master/CHANGELOG.md#1300--2020-10-20
postcss/postcss-import#432