Update `postcss` to v8 (#9209)
* Update `postcss` v8
* Parser custom properties
* Improve logic
* Update lint script
* Fix node location
* Fix comments inside
* Add a test
* Test on `less` and `scss` too
* Fix empty prop print
* Clean
* Update `postcss`
* Exclude `regenerator`
* Test `no-semi`
* Add changelog
* Typo
* Apply review suggestion
* Style
* Simplify logic
* Remove error test, we don't throw on this anymore
* Style
* Update postcss
* Update postcss
