Define Data Collections used in the spec

[benjie]

The spec uses the terms "set", "list" and "map" in many places. This PR defines these terms and their conformance requirements.

This is a change extracted from #1063 as discussed at last night's WG meeting

This is PR 1 of 3 in the "data collections" trilogy stack; next are:

2. https://github.com/graphql/graphql-spec/pull/1092
3. https://github.com/graphql/graphql-spec/pull/1063

cc @graphql/tsc

(Replacement of #1091; to do PR stack with #1092 and #1063)

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[benjie]

Hopefully my latest edit factors in @andimarek and @martinbonnin's feedback.