feat!: enable splitChunks by default even in development mode (#3863)

BREAKING CHANGE:
This changes the output directory structures for development mode
(app.js -> index.js + chunk-common.js + chunk-vendors.js).
By enabling splitChunks by default, the memory usage of webpack may be
greatly reduced for large multi-page projects.

This commit fixes #3838
May also fix #2991

Author: haoqunjiang

packages/@vue/cli-service/lib/config/app.js

@@ -24,7 +24,7 @@ module.exports = (api, options) => {
     const outputDir = api.resolve(options.outputDir)
 
     // code splitting
-    if (isProd && !process.env.CYPRESS_ENV) {
+    if (process.env.NODE_ENV !== 'test') {
       webpackConfig
         .optimization.splitChunks({
           cacheGroups: {