fix: make vue-loader-v16 an optional dependency, avoid crashing npm 5 (#5718)

fixes #5715

Author: haoqunjiang

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

@@ -72,14 +72,16 @@
     "thread-loader": "^2.1.3",
     "url-loader": "^2.2.0",
     "vue-loader": "^15.9.2",
-    "vue-loader-v16": "npm:vue-loader@^16.0.0-beta.3",
     "vue-style-loader": "^4.1.2",
     "webpack": "^4.0.0",
     "webpack-bundle-analyzer": "^3.8.0",
     "webpack-chain": "^6.4.0",
     "webpack-dev-server": "^3.11.0",
     "webpack-merge": "^4.2.2"
   },
+  "optionalDependencies": {
+    "vue-loader-v16": "npm:vue-loader@^16.0.0-beta.3"
+  },
   "peerDependencies": {
     "@vue/compiler-sfc": "^3.0.0-beta.14",
     "vue-template-compiler": "^2.0.0"