nightly-2024-09-16: feat: add `Expr::as_for` and `Expr::as_for_range` (#6039)

# Description

## Problem

Part of #5668

## Summary


## Additional Context



## Documentation

Check one:
- [ ] No documentation needed.
- [x] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist

- [x] I have tested the changes locally.
- [x] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.