Fix babel "loose" warnings

This fixes the following warnings emitted by webpack:

```
Though the "loose" option was set to "false" in your @babel/preset-env config, it will not be used for @babel/plugin-proposal-private-methods since the "loose" mode option was set to "true" for @babel/plugin-proposal-class-properties.
The "loose" option must be the same for @babel/plugin-proposal-class-properties, @babel/plugin-proposal-private-methods and @babel/plugin-proposal-private-property-in-object (when they are enabled): you can silence this warning by explicitly adding
  ["@babel/plugin-proposal-private-methods", { "loose": true }]
to the "plugins" section of your Babel config.
```

This matches the new default babel config generated by webpacker:
rails/webpacker#3016

babel.config.js

@@ -47,6 +47,12 @@ module.exports = function(api) {
           loose: true
         }
       ],
+      [
+        require('@babel/plugin-proposal-private-methods').default,
+        {
+          loose: true
+        }
+      ],
       [
         require('@babel/plugin-proposal-object-rest-spread').default,
         {