Member deletion

Currently in the language reference the following solution for removing fields is recommended:

```pkl
// https://pkl-lang.org/main/current/language-reference/index.html#transforming-objects

dodo {
name = "Dodo"
extinct = true
}

dodo
.toMap()
.remove("name")
.toDynamic()
```

Instead, create a built-in singleton value `absent` (similar to `null`), which would remove the corresponding member from the object:

```
dodo {
name = "Dodo"
extinct = true
}

dodoWithoutName = (dodo) {
name = absent
}
```

Start with the language reference's “transforming objects” section and the linked example. Determine how a built-in `absent` singleton should remove members during object transformations, then verify that the documented example expresses the intended result.

Written by the indexing model from the issue text.