chore!: drop support of NPM 5 (#6051)

Author: haoqunjiang

package.json

@@ -72,16 +72,14 @@
     "thread-loader": "^3.0.0",
     "url-loader": "^4.1.1",
     "vue-loader": "^15.9.2",
+    "vue-loader-v16": "npm:vue-loader@^16.0.0-beta.7",
     "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.7"
-  },
   "peerDependencies": {
     "@vue/compiler-sfc": "^3.0.0-beta.14",
     "vue-template-compiler": "^2.0.0"