Replace ExecuteSelectionSet with ExecuteGroupedFieldSet

[benjie]

Essentially this PR raises the concept of "grouped field sets" to be a top-level concept, and replaces the ExecuteSelectionSet method with ExecuteGroupedFieldSet (which essentially just drops the first line of ExecuteSelectionSet, which was responsible for producing the grouped field set to be executed). It then refactors the rest of the spec to accommodate this change, reducing the repetition in ExecuteQuery, ExecuteMutation and ExecuteSubscriptionEvent; and removing MergeSelectionSets (which generated a "virtual" selection set to accomodate the ExecuteSelectionSet method), instead adding a CollectSubfields algorithm which generates a grouped field set directly, ready for execution.

I extracted this common refactoring from a number of my attempts to write spec changes for the @defer and @stream directives - it turns out that this refactoring of the spec was always needed as a base for my changes. Similarly, @yaacovCR found similar in his attempts to address this same problem, and raised #999 extracted from his solution. This PR was introduced independently of #999 (other than using the CollectSubfields algorithm name) however there is significant alignment, so @yaacovCR suggested that I raise it as an alternative PR.

It may be easier to review this PR in "split" view rather than "unified" view.

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[benjie]

(Rebased on main)

[benjie]

(Rebased on main)

[Keweiqu]

@benjie we need to formally define "grouped field set" in Section 2 "Language". You might have done so in a prior PR that I missed. Here is the link to us defining "section sets" in section 2 https://spec.graphql.org/draft/#sec-Selection-Sets

[benjie]

This is up-to-date and ready-to-go again.

@yaacovCR or @JoviDeCroock; please can you confirm this latest text still aligns with GraphQL.js.

@Keweiqu Since a "grouped field set" is only part of execution and doesn't have an expression in the language I've not defined it in chapter 2, but in chapter 6 instead. (Here's where the current spec first introduces the term: https://spec.graphql.org/draft/#sel-FANRFABAB5EnhJ )