Adds Nix flake

[macalinao]

Description

Adds a Nix flake, making it easier for other Nix users to include these packages in their flakes.

Checklist

• [x] tested locally
• [x] added automated tests
• [x] updated the docs
• [x] updated the changelog

[macalinao]

Don't think there's any docs or changelog to update.

[asymmetric]

Hey @macalinao, thanks for this! I hadn't seen your PR and ended up doing this in a separate PR, so closing this, but thanks anwyay for taking the time!