Elm Development Environment with Nix

1st chunk of `doc/packages/elm.section.md`

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# Elm {#sec-elm}

To start a development environment, run:

```ShellSession
nix-shell -p elmPackages.elm elmPackages.elm-format
```

To update the Elm compiler, see `nixpkgs/pkgs/development/compilers/elm/README.md`.

To package Elm applications, [read about elm2nix](https://github.com/hercules-ci/elm2nix#elm2nix).

Title: Elm Development Environment with Nix

Summary

This chunk provides instructions for setting up an Elm development environment using `nix-shell` with `elmPackages.elm` and `elmPackages.elm-format`. It also directs users to `nixpkgs/pkgs/development/compilers/elm/README.md` for updating the Elm compiler and links to `elm2nix` for packaging Elm applications.