upgraded all npm packages (webpack 5, eslint 7, etc) and activated eslint

build/webpack.config.dev.js

@@ -1,7 +1,7 @@
 'use strict'
 
 const webpack = require('webpack')
-const merge = require('webpack-merge')
+const { merge } = require('webpack-merge')
 const baseConfig = require('./webpack.config.base')
 const MiniCssExtractPlugin = require("mini-css-extract-plugin");