Fix babel "loose" warnings

[mattbrictson]

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 suggested by webpacker:
rails/webpacker#3016

Steps to verify

1. Check out the main branch.
2. Delete node_modules and public/packs*.
3. Run bin/setup.
4. Run bin/webpack.
5. You'll see lots of warnings.
6. Check out this PR's branch.
7. Repeat steps 2, 3, 4.
8. You shouldn't see any warnings.