Distribution: Add nix flake method

[aster-void]

Summary

This PR adds Nix Flake support to the nbt-to-mcstructure project, enabling users to run the tool using the Nix package manager without manually installing dependencies.

Benefits

• Zero-setup installation: Users can run the tool directly without manually cloning, installing Python and installing dependencies

Usage

Users can now run the converter with a single command:

nix run github:JaylyDev/nbt-to-mcstructure

The traditional Python installation method remains fully supported.

[aster-void]

You can test this functionality by running:

nix run github:aster-void/nbt-to-mcstructure?ref=dist/nix-flake