Resolve nixpkgs evaluation warnings and add flake.lock

[Luytan]

Hello,
When evaluating this flake, it throws several evaluation warnings caused by deprecated xorg packages:

evaluation warning: The xorg package set has been deprecated, 'xorg.libxcb' has been renamed to 'libxcb'
evaluation warning: The xorg package set has been deprecated, 'xorg.libX11' has been renamed to 'libx11'
evaluation warning: The xorg package set has been deprecated, 'xorg.libXcursor' has been renamed to 'libxcursor'
evaluation warning: The xorg package set has been deprecated, 'xorg.libXrandr' has been renamed to 'libxrandr'
evaluation warning: The xorg package set has been deprecated, 'xorg.libXi' has been renamed to 'libxi'

This PR fixes this by renaming the xorg.* packages to their modern name

I also added a flake.lock file, which provide which provides exact reproducibility for Nix user