NixPkgs version badge

### 📋 Description

I see the Hackage badge: https://shields.io/badges/hackage-version
Here is an example of it in use: https://github.com/haskell/haskell-language-server?tab=readme-ov-file#haskell-language-server
I assume that functions by making a call to Hackage Server.

It would be very cool to have a similar badge for NixPkgs. This is not limited to Haskell.

### 🔗 Data

* `nix search` tool; this pulls from NixPkgs directly:
https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-search
```
$ nix search nixpkgs#haskellPackages '^haskell-language-server'
* legacyPackages.x86_64-linux.haskellPackages.haskell-language-server (2.5.0.0)
LSP server for GHC
```

* Keeping a local copy of NixPkgs Git repo and searching it:
```
$ nix search .#haskellPackages '^haskell-language-server'
* legacyPackages.x86_64-linux.haskellPackages.haskell-language-server (2.5.0.0)
LSP server for GHC
```

* another example:
```
$ nix search .#haskell-language-server
* legacyPackages.x86_64-linux.haskell-language-server (2.5.0.0)
LSP server for GHC
```

* another example:
```
$ nix search .#firefox
* legacyPackages.x86_64-linux.firefox (123.0.1)
A web browser built from Firefox source tree
```

* Less usable, UI search
https://search.nixos.org/packages?channel=unstable&from=0&size=50&sort=relevance&type=packages&query=haskell-language-server

### 🎤 Motivation

Encourage the use of packages hosted on NixPkgs, and also encourage contribution to update the NixPkgs

Thanks

Start by comparing the existing Hackage version badge at shields.io/badges/hackage-version with the proposed NixPkgs behavior. Review the documented nix search examples and the NixOS package search URL to determine the supported package lookup and version source. Done means a NixPkgs version badge can identify a package and display its current version reliably.

Written by the indexing model from the issue text.