fix: Fix __typename fields on abstract selections not being exact

[kitten]

Summary

When a __typename field is used on an abstract type’s selection set its value type wouldn’t be exact and instead a union of all possible types unless all possible types have selections containing a __typename field themselves.

{
  test { # possible types: A / B
    __typename
    ... on B {
      field
    }
  }
}

The above would result in an incorrect inference since B’s selection set doesn't contain __typename itself:

const before: {
  test: {
    __typename: 'A' | 'B';
  } | {
    __typename: 'A' | 'B';
    field: unknown;
  };
};

const after: {
  test: {
    __typename: 'A';
  } | {
    __typename: 'B';
    field: unknown;
  };
};

Set of changes

• Move __typename field handling to the selection handler that handles all possible types separately
• Remove field handling entirely in favour of always handling fields per possible type
• Refactor and rename utilities as appropriate

[changeset-bot]

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Latest commit: e895c89

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