fix: __typename should succeed on root fields

[mattjohnsonpint]

Description

We already support returning __typename fields on user-defined types, such as:

query {
    people {
        __typename
        firstName
        lastName
        fullName
    }
}

... which would give "Person" for the __typename field.

However, asking for __typename on the root Query or Mutation currently fails, and it should succeed. This is important because some GraphQL clients add __typename to every field in a query, including the root fields.

In other words, these should succeed:

query {
   __typename
}

mutation {
   __typename
}

Checklist

• Code compiles correctly and linting passes locally
• For all code changes, an entry added to the CHANGELOG.md file describing and linking to this PR