Local variables inside for loops yield an error

Code:

```pkl
hidden xs = new Listing { 1 2 3 }

object {
for (x in xs) {
local plusOne = x + 1
plusOne
}
}
```

On evaluation yields the following error:

```
A for-generator cannot generate object properties (only entries and elements).
```

Instead, expected to produce object with entries `2 3 4`.

Version:

```
% pkl --version
Pkl 0.26.0 (macOS 14.3.1, native)
```

Start by reproducing the issue with the provided Pkl snippet on the reported version, then trace the compiler handling of for-generators and local variables inside their bodies. The fix is done when the example evaluates successfully and produces entries 2, 3, and 4 without the object-property error.

Written by the indexing model from the issue text.