Add development flake

[crertel]

Adds a development flake for folks working on this using Nix.

Description

Adds a spec for Nix development.

Motivation and Context

Simplifies development wrangling if you're using Nix.

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any feature
  but make things better)

Checklist

• Check other PRs and make sure that the changes are not done yet.