Forbid @skip and @include directives in subscription root selection

[benjie]

This is an implementation of https://github.com/graphql/graphql-spec/pull/860

The spec calls for a separate CollectSubscriptionFields algorithm due to executing without variableValues; but I figured that maintenance would be easier with the algorithms synchronized.

Please feel free to make any changes you need to this PR; it's just to get the ball rolling.

Related GraphQL WG action item (from 2021!): https://github.com/graphql/graphql-wg/issues/695

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[benjie]

GraphQL v17 would be a good time to add this "breaking change" @graphql/graphql-js-reviewers