Update dependency postcss to v8.1.0

[renovate-bot]

This PR contains the following updates:

Package	Type	Update	Change
postcss (source)	devDependencies	minor	8.0.9 -> 8.1.0

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Release Notes

postcss/postcss

v8.1.0

Compare Source

PostCSS 8.1 fixed new visitor API from the 8.0 release.

We fixed Root and RootExit re-calling on children's changes. And now visitors will visit the parent again if nested children were changed.

We added Once and OnceExit events, which will not be called again on node changes. You can use them to lint files or collect statistics:

module.exports = {
  postcssPlugin: 'postcss-linter',
  OnceExit (root) {
    lint(root)
  }
}
module.exports.postcss = true

We updated Migration guide according to new changes.

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