Add `Starlet` to the Users section in `users.md`

[vt128]

This PR aims to add Starlet to the list of Starlark users.

Starlet is a Go wrapper for the official Starlark in Go project, designed to enhance the Starlark scripting experience with powerful extensions and enriched wrappers. It offers an intuitive API for executing Starlark scripts, managing global variables, loading modules, controlling the script execution flow, and handling script outputs. Besides, it provides a simplified data exchange mechanism between Go and Starlark types.

Moreover, Starlet includes a set of custom modules and libraries that extend the functionality of the Starlark language, covering a wide range of use cases, such as file manipulation, HTTP client, JSON/CSV handling, and more.

The addition of Starlet to the list of Starlark users would provide a valuable resource for the community, as it illustrates a practical use of Starlark and offers a toolset that can simplify and enhance the Starlark scripting experience for developers.

Please let me know if there are any questions or if further information is needed.

Thanks!