Enhance Flake: Add Package Output and DevShell

This repository does contain a `flake.nix` file, but it currently does not expose itself as a Nix package.
I will be working on a PR in the coming days.
The PR will include the `devShell` environment and package definitions.

@eswar2001 I noticed the use of bash functions in the `shellHook` section [here](https://github.com/juspay/code-as-data/blob/c573e85c397e11f41c9f01a29717543bb70017b4/flake.nix#L75-L103) to start and stop Postgres.
Could you please clarify what exactly needs to be done on the Postgres part?
I'm planning to rework that part using [`services-flake`](https://github.com/juspay/services-flake).

Also, will be using [uv2nix](https://github.com/pyproject-nix/uv2nix)

Review flake.nix, especially the shellHook at lines 75–103, along with the planned package and devShell definitions. Clarify the PostgreSQL setup before evaluating services-flake and uv2nix. Done means the flake exposes the requested package and devShell, with the PostgreSQL approach resolved.

Scritto dal modello di indicizzazione a partire dal testo della issue.