Bump target to es2018 to address build failure,

see jupyterlab/jupyterlab#14029

Author: krassowski

packages/labextension/tsconfig.json

@@ -17,7 +17,7 @@
     "rootDir": "src",
     "strict": true,
     "strictNullChecks": true,
-    "target": "es2017",
+    "target": "es2018",
     "types": []
   },
   "include": ["src/**/*"]