package.json

// ...
"paths": {
  "conf": "./config",
  "tasks": "./tasks"
},
"scripts": {
    "build": "npm-run-all --parallel build:*",
    // you cannot write comments in package.json (or any json, so don't do this! ;)
    // but to explain this, $npm_package_paths_configs refers to what you have as value in the
    // paths property up there 
    "build:general": "webpack-cli --config \"$npm_package_paths_conf/webpack.config.js\"",
    "build:js": "webpack-cli --config \"$npm_package_paths_conf/webpack.js.config.js\"",
    "build:css": "webpack-cli --config \"$npm_package_paths_conf/webpack.css.onfig.js\"",
    // ...
    "clean": "node $NODE_DEBUG_OPTION \"$npm_package_paths_tasks/clean\"",
  ...
}
  

webpack.config.js

const jsConfig = require('./webpack.js.config');
const cssConfig = require('./webpack.css.config');
const merge = require('webpack-merge');

const generalConfig = merge([
  jsConfig,
  cssConfig
]);

module.exports = generalConfig;