npm i jquery bootstrap@3 -S

webpack.config.js

const webpack = require('webpack')

module.export = {
  ...
  plugins: [
        new webpack.ProvidePlugin({
            jQuery: 'jquery',
            $: 'jquery'
        })
    ]
}

main.js

import 'bootstrap/dist/js/bootstrap.js'
import 'bootstrap/dist/css/bootstrap.css'