Transpile nullish coalesing and optional chaining operators

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

babel.config.js

@@ -19,6 +19,10 @@ const plugins = [
   '@babel/plugin-proposal-private-methods',
   // See: https://gitlab.com/gitlab-org/gitlab/-/issues/229146
   '@babel/plugin-transform-arrow-functions',
+  // See: https://gitlab.com/gitlab-org/gitlab/-/issues/336216
+  '@babel/plugin-proposal-optional-chaining',
+  // See: https://gitlab.com/gitlab-org/gitlab/-/issues/336216
+  '@babel/plugin-proposal-nullish-coalescing-operator',
   'lodash',
 ];