Add nix development environment

[naomieow]

Add support for a nix development environment that works with or without the use of flakes. This will make it easier for people who use nix (such as myself) to contribute to the project.

This PR also adds files used by/generated by the direnv project as this is quite often used in conjunction with nix development environments.

[Copilot]

Pull Request Overview

This PR adds Nix development environment support to make it easier for Nix users to contribute to the project. It provides both traditional Nix shell and modern Nix flakes support.

• Adds shell.nix for traditional Nix users with Gleam development dependencies
• Adds flake.nix for modern Nix flakes users with cross-platform support
• Enables direnv integration for automatic environment loading

Reviewed Changes

Copilot reviewed 2 out of 4 changed files in this pull request and generated 1 comment.

File	Description
shell.nix	Defines Nix shell environment with Gleam, Erlang 28, and rebar3 dependencies
flake.nix	Provides flakes-compatible wrapper with multi-platform support for the shell environment

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[cyteon]

lgtm, and cant rly test cause i dont use nix soooo