[2020-10-01] Add validation rule to forbid introspection fields in the root of a GraphQL subscription

[benjie]

__typename at the root of a subscription operation throws an error. It doesn't make sense to support it. It was agreed that since it already doesn't work, adding an explicit validation rule to forbid it would make sense and be "non-breaking".

Original PR here: graphql/graphql-spec#776

TODO:

• PR to spec

• PR to GraphQL.js

• assignee(s): @benjie

• source: https://github.com/graphql/graphql-wg/blob/master/notes/2020-10-01.md#__typename-and-subscriptions-5m-benjie

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[benjie]

@IvanGoncharov Given GraphQL.js is in feature freeze; should I hold off on this PR to GraphQL.js?

[benjie]

Spec PR updated, GraphQL.js PR filed.