Hi !

This PR adds notmuch's Vim plugin and Ruby bindings. The Ruby bindings are required by the Vim plugin.
The first commit is a bit unrelated, it removes emacs from the build dependencies when withEmacs is false. I don't even want this on my computer ! (kidding, it was just an easy fix)

The Vim plugin is enabled by default because it consists only of small static files. The ruby bindings are not big either and ruby is already added as a dependency. Also emacs is enabled by default, so it's fair.

The way I hook the ruby bindings doesn't seem right. I couldn't find examples in nixpkgs and I think bundlerEnv cannot be used here. Is here a better way ?

The ruby bindings have a dependency on the gem mail but it is optional and loaded at runtime, so I haven't included it.

There is a related issue: #43965

Motivation for this change

Make the Vim plugin available from nixpkgs and to use it like this:

let
  notmuch = (pkgs.notmuch.override {
    withEmacs = false;
  }).overrideAttrs (o: {
    doCheck = false; # Too slow
  });

  # Shadows vim, loading the notmuch plugin and a local vimrc
  notmuch_vim = { vimrc }:
    pkgs.runCommand "notmuch-wrapped-vim" {
      buildInputs = [ notmuch ];
    } ''
      # No absolute path to vim on purpose
      mkdir -p "$out/bin"
      cat <<EOF > "$out/bin/vim"
      #!/usr/bin/env bash
      this="$out"
      PATH=\''${PATH//\$this/} # Remove this package from the PATH
      exec vim -c "set packpath+=\$this/vim" -c "packadd notmuch" -c "source ${vimrc}" "\$@"
      EOF
      chmod +x "$out/bin/vim"
      plugin_out="$out/vim/pack/plugins/opt" # Vim plugins directory structure
      mkdir -p "$plugin_out"
      ln -sf "${notmuch}/share/vim" "$plugin_out/notmuch"
    '';

I got a bit creative here, I'll probably use something saner :D

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.