Run npm dedupe

`npm` was installing two different versions of `postcss`, one in the root `node_modules` and on in the `shared/tasks` `node_modules`. Those had different versions (likely due to `stylelint-order` requiring `^8.5.8`).

Because the versions didn't match, TypeScript couldn't reconciliate the types in `shared/tasks/styles.mjs`. After `npm dedupe` there's only one package in the root `node_modules` and TypeScript is happy.

Author: romaricpascal