chore!: upgrade terser-webpack-plugin to 2.x (#4676)

BREAKING CHANGE:
The only real breaking change is https://github.com/webpack-contrib/terser-webpack-plugin/issues/129,
which is not likely to affect normal users.

The rest are default option values changes, which are already covered in
the default provided `terserOptions` of Vue CLI.

Note that here we choose to disable `extractComments` by default, as I
don't see enough data showing such license comments taking too much
space. The extra LICENSE file may also be confusing to some users.

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

@@ -37,5 +37,6 @@ module.exports = options => ({
   },
   sourceMap: options.productionSourceMap,
   cache: true,
-  parallel: options.parallel
+  parallel: options.parallel,
+  extractComments: false
 })

packages/@vue/cli-service/package.json

@@ -69,7 +69,7 @@
     "source-map-url": "^0.4.0",
     "ssri": "^6.0.1",
     "string.prototype.padend": "^3.0.0",
-    "terser-webpack-plugin": "^1.3.0",
+    "terser-webpack-plugin": "^2.1.2",
     "thread-loader": "^2.1.2",
     "url-loader": "^2.1.0",
     "vue-loader": "^15.7.0",