Provide flake.nix

• add a flake.nix

• update nix/Cargo.nix

• refactor nix/default.nix for flake compatibility and to make it a bit more tidy

  • put mozilla overlay into its own file (nix/rustPkgs.nix)
  • move a bunch of utility functions into nix/utils.nix

• add flake usage documentation to nix/README.md

• By opening this merge request, you agree to release your code and all other changes under the GPL 3 license and to abide by the terms set by this license.

• Migrations have been added if applicable

• Significant changes of this merge request have been added to the changelog.