fix: webpack-sources is an optinal peer dependency

It is only required when we are using webpack 4

Author: haoqunjiang

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

@@ -107,6 +107,9 @@
     },
     "vue-template-compiler": {
       "optional": true
+    },
+    "webpack-sources": {
+      "optional": true
     }
   },
   "devDependencies": {