Pkl Error: Cannot render value of type `Listing` as Properties.

I have the schema as following:

```
server: Server

class Server {
endpoints: Listing
}

class Endpoint {
name: String
port: UInt16(isBetween(1024, 65535))
}
```

and configuration:

```
server {
endpoints {
new {
name = "localhost"
port = 8080
}
new {
name = "demo"
port = 8080
}
}
}
```

when run `pkl eval -f properties application.pkl` and got the stackstrace:

```
– Pkl Error ––
Cannot render value of type `Listing` as Properties.
Value: new Listing { ?; ? }
```

I think pkl properties render could adopt list style.

```
foo.bar[0]="a"
foo.bar[1]="b"
foo.demo[0].host="localhost"
foo.bar[0].port=8080
```

Reproduce the failure with `pkl eval -f properties application.pkl` using the schema and configuration in the issue, then trace the Properties rendering entry point. Determine how Listing values are handled and define indexed output such as `foo.bar[0]="a"`; done means listings render without the current error and their nested values use stable indexes.

Written by the indexing model from the issue text.