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

[martinbonnin]

Follow up from discord

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: graphql/graphql-js#4415