fix(plugin-legacy): use terser as the default minifier (#5168)

Author: haoqunjiang

packages/playground/legacy/vite.config.js

@@ -10,9 +10,6 @@ module.exports = {
   ],
 
   build: {
-    // make tests faster
-    minify: false,
-
     rollupOptions: {
       output: {
         chunkFileNames(chunkInfo) {

packages/plugin-legacy/index.js

@@ -96,6 +96,14 @@ function viteLegacyPlugin(options = {}) {
     name: 'vite:legacy-generate-polyfill-chunk',
     apply: 'build',
 
+    config() {
+      return {
+        build: {
+          minify: 'terser'
+        }
+      }
+    },
+
     configResolved(config) {
       if (!config.build.ssr && genLegacy && config.build.minify === 'esbuild') {
         throw new Error(