mutate-webpack-config.js

export function loadConfigModule(filename) {
  // sub your own
  return require(filename);
}

/**
 *  Modify a Webpack configuration by applying a mutator module/function.
 *  @param {Object} options
 *  @param {Object} options.config    Webpack configuration object to modify in-place
 *  @param {Object} options.module    A configuration function of the form `(config, env)=>`,
 *                                    or an Object with a `webpack` method of that form.
 *  @param {String} [options.target]      "client" for browser bundles, "server" for Node/SSR/prerendering
 *  @param {Boolean} [options.production] Defaults to `process.env.NODE_ENV==='production'`
 *  @param {Boolean} [options.watch]      Whether the bundle is being built for HMR/watch usage
 *  @param {Object} [options.env]         Environment values to expose as options to the config function
 *  @param {Object} [options.helpers]     Utility functions to pass as a 3rd arg to the config function
 *                                        Example: github.com/developit/preact-cli/blob/master/src/lib/webpack/transform-config.js#L29
 */
export function modifyConfig({
  config,
  module,
  target = 'client',
  production = process.env.NODE_ENV === 'production',
  watch = false,
  env = {},
  helpers = {}
}) {
  const webpackMutator = module.webpack || module;
  if (typeof webpackMutator !== 'function') {
    throw Error(`Webpack configuration plugin must be a function of the form (config, { target }) => void`);
  }
  
  const ssr = target === 'server';
  const options = {
    production,
    watch,
    target,
    ...env,
    // Compat aliasdes for Next.js, Preact CLI, etc
    ssr,
    isProd: production,
    isWatch: watch,
    isServer: ssr,
  };
  
  webpackMutator(config, options, helpers);
  
  return config;
}

Whether it's in webpack or not, webpack-merge seems like a good starting point.