Postcss 8 and TypeScript typings

Breaking changes:

• postcss 8 is now required
• postcss is now a peer dependency, run npm install postcss or yarn add postcss to ensure it is in your project
• TypeScript typings are now provided, these typings build in postcss's own typings and require TypeScript 4 or higher

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postcss-combine-duplicated-selectors now leverages the postcss single scan visitor API.
This has a few implications:

• With new API, all plugins can share a single scan of the CSS tree. It makes CSS processing up to 20% faster.
• Plugins will re-visit changed nodes to reduce compatibility issues between plugins. Now the order of plugins in your PostCSS config will be less important.

source: https://github.com/postcss/postcss/releases/tag/8.0.0