Document literal should return an error when referring to field not yet declared

Example:

```sql
genji> create table foo;
genji> insert into foo values {a: b+1, b: 1};
genji> select * from foo;
{
  "a": null,
  "b": 1
}
```

The document literal is evaluated from left to right. When `b + 1` is being evaluated, it is not yet declared and thus evaluate to NULL.
We should return an error instead.