A field resolver function.
Generic with unknown defaults, matching graphql-js's own
GraphQLFieldResolver<TSource, TContext, TArgs> shape. The defaults keep
every existing resolver assignable; a narrow annotation —
FieldResolver<IssueRow, DefaultGraphqlContext, { id: string }> — assigns
cleanly too, which under strictFunctionTypes was impossible while the
parameters were non-generic unknown (X6-4).
The resolver context (see DefaultGraphqlContext).