build($core): bump babel and core-js version (close #2046) (#2165)

packages/@vuepress/core/package.json

@@ -23,8 +23,8 @@
     ">1%"
   ],
   "dependencies": {
-    "@babel/core": "^7.0.0",
-    "@vue/babel-preset-app": "^3.1.1",
+    "@babel/core": "^7.8.4",
+    "@vue/babel-preset-app": "^4.1.2",
     "@vuepress/markdown": "^1.3.0",
     "@vuepress/markdown-loader": "^1.3.0",
     "@vuepress/plugin-last-updated": "^1.3.0",
@@ -36,6 +36,7 @@
     "chokidar": "^2.0.3",
     "connect-history-api-fallback": "^1.5.0",
     "copy-webpack-plugin": "^5.0.2",
+    "core-js": "^3.6.4",
     "cross-spawn": "^6.0.5",
     "css-loader": "^2.1.1",
     "file-loader": "^3.0.1",