Fixes: https://github.com/astral-sh/ty/issues/92
## Summary
We currently get a `invalid-argument-type` error when using
`dataclass.fields` on a dataclass, because we do not synthesize the
`__dataclass_fields__` member.
This PR fixes this diagnostic.
Note that we do not yet model the `Field` type correctly. After that is
done, we can assign a more precise `tuple[Field, ...]` type to this new
member.
## Test Plan
New mdtest.
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Co-authored-by: David Peter <mail@david-peter.de>
