update webpack and related versions

- https://webpack.js.org/migrate/5/
    ```
    BREAKING CHANGE: webpack < 5 used to include polyfills for node.js core modules by default.
    This is no longer the case. Verify if you need this module and configure a polyfill for it.

    If you want to include a polyfill, you need to:
            - add a fallback 'resolve.fallback: { "buffer": require.resolve("buffer/") }'
            - install 'buffer'
    If you don't want to include a polyfill, you can use an empty module like this:
            resolve.fallback: { "buffer": false }
    ```
- https://github.com/postcss/postcss/wiki/PostCSS-8-for-end-users

Author: TimidRobot