Add nix devshell

[busyboredom]

Description

Add a flake.nix providing a nix devshell for easy dev environment setup.

Checklist

• [x] Link relevant issue(s).
• [x] Manually test the change.
• [ ] Ensure sufficient automated test coverage.
• [x] Update the README.md if necessary.
• [x] Update the CHANGELOG.md if necessary.