Generate language server primitives in helix-cogs Nix output

[clo4]

This PR exposes the generated primitives for the language server to Nix users. As far as I can tell, this wasn't possible using the flake before - please let me know if I'm wong about that!

The original cogs directory remains unchanged, but a new steel-language-server directory is added to the helix-cogs output. Using Home Manager, a user could symlink that to STEEL_LSP_HOME, or even do something like this with a nested LSP home.

{ inputs, config, ... }:
{
  xdg.dataFile.steel.source = inputs.helix.packages.${system}.helix-cogs;
  home.sessionVariables.STEEL_HOME = "${config.xdg.dataHome}/steel";
  home.sessionVariables.STEEL_LSP_HOME = "${config.xdg.dataHome}/steel/steel-language-server";

  # Uncommenting the following line also allows other scm files to be added since
  # the files are copied recursively instead of symlinking the directory itself.
  # xdg.dataFile.steel.recursive = true;
}

I'm open to changing this. I considered splitting this into two outputs, but found the simplicity of this much nicer since the output of the helix-cogs derivation is just files. Please let me know if you'd prefer a different structure or some renaming. As it is, I'm happy with this.