[2020-12-03] Complete GraphQL.js implementation of subscription introspection validation rule

[benjie]

graphql/graphql-spec#776 was made RFC 1 (Proposal) during the WG call; next month we should aim to advance it to RFC 2 (Draft). The main task for this is to advance this GraphQL.js PR: graphql/graphql-js#2861

• assignee(s): @benjie
• source: https://github.com/graphql/graphql-wg/blob/master/notes/2020-12-03.md#rfc-__typename-is-not-valid-at-subscription-root-10m-benjie

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Note: Action Item issues are reviewed and closed during Working Group
meetings.

[benjie]

Implementation is now passing CI checks; assigned to @IvanGoncharov for further review:

graphql/graphql-js#2861

(Note: this also fixes graphql/graphql-js#2715 which seems to be a bug in the reference implementation's one-field subscription validation rule?)