Request to Add pkl (Configuration as Code Language) Package to NixOS

### Issue Description

#### Problem

The `pkl` package, a Configuration as Code language developed by Apple, is not currently available in the NixOS packages.

#### Desired Outcome

Include the `pkl` package in NixOS packages.

#### Package Information

- **Package Name:** pkl
- **GitHub Repository:** [apple/pkl](https://github.com/apple/pkl)

#### Additional Context

[pkl](https://github.com/apple/pkl) is a Configuration as Code language with rich validation and tooling. Adding it to NixOS packages would benefit users involved in configuration management and development workflows.

### Environment Information

- **NixOS Version:** 23.11 or current
- **Nixpkgs Channel:** unstable

#### Reference Links

1. [NixOS Website](https://nixos.org/)
2. [NixOS Package Search - pkl](https://search.nixos.org/packages?channel=unstable&from=0&size=50&sort=relevance&type=packages&query=pkl)

Thank you for considering this request.

Begin with the apple/pkl repository and the linked NixOS package search to determine the package's build requirements and the appropriate packaging entry point. Done means pkl is available in the NixOS packages and can be found through the package search.

Written by the indexing model from the issue text.