[graphiql] Make GraphiQL robust to new built-in types being added

Follow up from [discord](https://discord.com/channels/625400653321076807/971515695881924621/1367379788376838144)

As we are investigating adding new built-in types to GraphQL, this breaks GraphiQL with the following error:


Looks like adding any type starting with `__` will trigger an error. This is problematic as it forbids evolution of the specification.

This happens when calling `validateSchema()` because the `@behaviour` is seen as a custom directive using a reserved type.

It's too late to change the past but fixing this sounds important for the soundness of the ecosystem for the many years to come.

Edit: looks like this will require some functionality in graphql-js: https://github.com/graphql/graphql-js/issues/4415