Add nix Flake
As nix does not have GXml in their packages, this PR adds a nix Flake which allows for building a nix package and developing GXml inside of a development environment with all dependencies set up and ready to go.
As nix does not have GXml in their packages, this PR adds a nix Flake which allows for building a nix package and developing GXml inside of a development environment with all dependencies set up and ready to go.