Misleading error message when mutable variable shadows outer function

The following code has a somewhat misleading error message: there is a `sum` which is mutable, it's just not something we can reach:

The code in question

```scala
def sum(xs: List[Int]): Int = {
var sum = xs.foreach { x =>
sum = sum + x
// ^ ~~~
}
sum
}
```

Of course when you use `res` for the mutable variable and keep `sum` for the function, you get `Could not resolve term res`, which is probably a better message in this case.

I have no real use case for this, I just introduced a typo by accident in https://effekt-lang.org/tour/variables and then wondered for a brief moment.

---

Here's the relevant compiler code:
https://github.com/effekt-lang/effekt/blob/afb9ef015b2f18e56958f43235c711f05612dc9e/effekt/shared/src/main/scala/effekt/Namer.scala#L516-L521