add a diagnostic for NewTypes with TypeVars in their base type

TODO from: https://github.com/astral-sh/ruff/pull/21157#pullrequestreview-3445518919

Something like this should be an error:

```py
T = TypeVar("T")
N = NewType("N", list[T])  # currently accepted, but not a "proper class"
```