feat(compiler-sfc): upgrade to postcss 8

[yyx990803]

close #2642

Note: not merging yet because we want to ensure compatibility with @vue/cli before releasing this.

[github-actions]

Size report

Path	Size
vue.global.prod.js	40.5 KB (0%)
runtime-dom.global.prod.js	26.81 KB (0%)
size-check.global.prod.js	16.18 KB (0%)

[kdankert]

Is there any estimate on when this is going to be released?

[bram-pkg]

Any update on this?

[Adeelasd]

any update?

[ghost]

any update?
https://github.com/vitejs/vite works with PostCSS 8 (alternative to @vue/cli)

[antfu]

@knightly-bot build this

[knightly-bot]

🌒 Knightly build enabled, release every night at 00:00 UTC ^{(skip if no change)}

📦 npm package

[LinusBorg]

Vue CLI is compatible with PostCSS, verified by @sodatea, so we should be good to merge here.