Add experimental option to enable terser loader (#7033)

packages/next-server/server/config.js

@@ -30,6 +30,7 @@ const defaultConfig = {
     ),
     ampBindInitData: false,
     exportTrailingSlash: true,
+    terserLoader: false,
     profiling: false,
     flyingShuttle: false,
     asyncToPromises: false

packages/next/build/webpack-config.ts

@@ -179,7 +179,7 @@ export default function getBaseWebpackConfig (dir: string, {dev = false, debug =
         new TerserPlugin({...terserPluginConfig,
           terserOptions: {
             safari10: true,
-            ...(selectivePageBuilding ? { compress: false, mangle: true } : undefined)
+            ...((selectivePageBuilding || config.experimental.terserLoader) ? { compress: false, mangle: true } : undefined)
           }
         })
       ] : undefined,
@@ -228,7 +228,7 @@ export default function getBaseWebpackConfig (dir: string, {dev = false, debug =
     // @ts-ignore this is filtered
     module: {
       rules: [
-        selectivePageBuilding && !isServer && {
+        (selectivePageBuilding || config.experimental.terserLoader) && !isServer && {
           test: /\.(js|mjs|jsx)$/,
           exclude: /\.min\.(js|mjs|jsx)$/,
           use: {