I think that the browser field is missing `url: false`.
I'm not completely sure that this is the right fix and also the npm package `url` does not implement the APIs used by postcss. 

steps to reproduce

```
mkdir postcss-browser 
cd postcss-browser 
mkdir src
echo 'import "postcss";' > src/index.js
npm init -y
npm install webpack webpack-cli postcss
npx webpack
```
This will emit error: 
```
ERROR in ./node_modules/postcss/lib/input.js 3:39-53
Module not found: Error: Can't resolve 'url' in 'C:\projects\postcss-web-bundle\postcss-browser\node_modules\postcss\lib'
```

Co-authored-by: Andrey Sitnik <andrey@sitnik.ru>