`extend:` in `nest:` generates a nasty error

[christopherswenson]

Repro test:

test('works in nest', () => {
  expect(
    'run: a -> { nest: n is { extend: { dimension: x is 1 }; group_by: x } }'
  ).toTranslate();
});

This should probably give an error like extend: is not allowed inside nest: rather than the crazy nextStructDef error that happens today.