cra-browserext-boilerplate: webpack.config.js structure

cra-browserext-boilerplate-webpack.config-structure.js

...
const paths = require('./paths');
...
module.exports = function(webpackEnv) {
  ...
  return {
    ...
    entry: [
      isEnvDevelopment &&
        require.resolve('react-dev-utils/webpackHotDevClient'),
      paths.appIndexJs,
    ].filter(Boolean),
    ...
  }
  ...
}