Our version of webpack's acorn is too old and throws errors when we are
not transpiling the optional chaining operators `?.` and nullish
coalescing operators `??` even though all our targeted browsers support
it.

Therefore we need to enable
[`@babel/plugin-proposal-optional-chaining`][0] and
[`@babel/plugin-proposal-nullish-coalescing-operator`][1] manually.

After we have updated to webpack@5, we can disable these plugins.

[0]: https://babeljs.io/docs/en/babel-plugin-proposal-optional-chaining
[1]: https://babeljs.io/docs/en/babel-plugin-proposal-nullish-coalescing-operator