__typename may not be included as a root field in a subscription operation.

[jangko]

• https://spec.graphql.org/draft/#sec-Type-Name-Introspection

  • fix the validation code
  • add more test cases for root field
  • add more test cases for inner fields

• graphql/graphql-spec#806

  • __typename is allowed for root of subscription

• graphql/graphql-spec#860

  • @Skip and @include should not allowed for subscription root