Update node 12 to node 16 in .babelrc

We only support node 16+ now, so we can target this version and get a
nicer build. See:

- #286
- #276

.babelrc

@@ -4,7 +4,7 @@
       "@babel/preset-env",
       {
         "targets": {
-          "node": "12"
+          "node": "16"
         }
       }
     ],