feat(gitui): add gitui to replace lazygit

[storopoli]

gitui now supports PGP-signed commits (see extrawurst/gitui#97).
gitui is written in Rust, and fast AF.
It beats lazygit in all benchmarks.

So let's add it.
However, we need to wait for a new version (>0.25.2) to land in nixpkgs.

Something like:

{ ... }:

{
  xdg.configFile = {
    # catppucin theme
    "gitui/theme.ron".source = builtins.fetchurl {
      url = "https://github.com/catppuccin/gitui/raw/ce0d073676dc96d005f7b91368e5cd565d840104/theme/mocha.ron";
      sha256 = lib.fakeSha256;
    };

    # vim keybindings
    "gitui/key_bindings.ron".source = builtins.fetchurl {
      url = "https://github.com/extrawurst/gitui/raw/8876c1d0f616d55a0c0957683781fd32af815ae3/vim_style_key_config.ron";
      sha256 = lib.fakeSha256;
    };
  };

  programs.gitui = {
    enable = true
  };
}

Also change the lazygit.nvim to nvim-gitui (or remove it altogether?)

[extrawurst]

It would be great to get more hands on the new gpg signing feature, as of today we have gitui-nightly releases, so plese give it a spin: https://fosstodon.org/@rusticorn/112169249117381783