test: test against typescript v3, fix webpack 5 errors

Author: haoqunjiang

package.json

@@ -88,17 +88,20 @@
     "stylus-loader": "^3.0.2",
     "sugarss": "^1.0.1",
     "ts-loader": "^4.2.0",
-    "typescript": "^2.8.3",
+    "typescript": "^3.9.5",
     "url-loader": "^1.0.1",
     "vue": "^2.5.16",
     "vue-server-renderer": "^2.5.16",
     "vue-template-compiler": "^2.5.16",
     "vuepress": "^0.14.2",
     "vuepress-theme-vue": "^1.1.0",
-    "webpack": "^4.1.0",
+    "webpack": "^5.0.0-0",
     "webpack-cli": "^3.2.0",
     "webpack-dev-server": "^3.1.1",
     "webpack-merge": "^4.1.2",
     "yorkie": "^1.0.3"
+  },
+  "resolutions": {
+    "webpack": "^5.0.0-0"
   }
 }