Migrate to ESM modules
    
    Changes:
    1. require nodejs v14+
    2. add linting for config folder
    3. dependencies:
      a) flow 0.163 is required to support import.meta.url (part of
         workaround to import JSON)
      b) remove whatwg-fetch polyfill - all modern browser support fetch
      c) remove postcss-safe-parser: CssMinimizerPlugin works well with
         built-in parser
    4. exceptions that require CommonJS (renamed to *.cjs)
      a) eslint 7.x - we cannot update to 8.x as our eslint-webpack-plugin
         for wp4 requires older version
      b) babel.app.config as it is  being used synchronously by eslint
    5. remove alternative client for displaying errors in WebpackDevServer
       - build-in solution was improved in newer versions