We already had some logic to prevent the typeof-symbol plugin from transpiling its own helper. It worked by generating the helper name and checking if the current path is inside a function or a variable declaration with that name.

However, it had a few problems:

1. It doesn't work with @babel/runtime, because the generated name is a unique identifier so it's guaranteed not to be equal to the _typeof helper name.
2. Sometimes, multiple names are generated (_typeof2 and _typeof3), causing circular dependencies. This is probably caused by running twice the plugin, but we can avoid failing in that case.

I initially hard-coded the helper name in the isUnderHelper logic to make it work with @babel/runtime, but:

1. It doesn't fix (2)
2. If @babel/runtime is bundled in an app/library by rollup, the function name might change
3. It doesn't work with minifiers.

I then decided to introduce a "@babel/helpers - typeof" directive, which has a few advantage:

1. We don't need to generate the helper code before checking if we are inside the helper, which would always lead to duplicated code when transpiling @babel/runtime/helpers/typeof
2. It fixes (2) and works with Rollup.

The directive still doesn't work with Terser and UglifyJS because they strip them by default. We could either suggest to whoever finds this bug to set the directives: false option in their minifier settings, or we could ask UglifyJS and Terser's maintainers if they could keep this specific directive in the minified output.

This PR makes it safe to transpile @babel/runtime, which should provide a solution for #9903