update to webpack 4

webpack/service-worker.config.js

@@ -3,15 +3,7 @@ const config = require('sapper/webpack/config.js');
 const webpack = require('webpack');
 
 module.exports = {
-	entry: {
-		'service-worker': './app/service-worker.js'
-	},
-	output: {
-		path: path.resolve(`.sapper`),
-		filename: '[name].js',
-		chunkFilename: '[name].[id].[hash].js'
-	},
-	plugins: [
-		!config.dev && new webpack.optimize.ModuleConcatenationPlugin()
-	].filter(Boolean)
+	entry: config.serviceworker.entry(),
+	output: config.serviceworker.output(),
+	mode: process.env.NODE_ENV
 };