programs/zsh: rewrite the zsh module

[nezia1]

This PR overhauls the zsh module API. Taking heavy inspiration from the HM API, as I could not find a better/cleaner way to do this.

This simplifies declaring plugins, and also allows for more flexibility when it comes to files that don't follow conventional plugin naming and directory structure.

This brings in breaking changes, as this completely changes the API. If you want me to add temporary renames so that it helps migrating to it, let me know.

An example usage (from my personal configuration):

...
  plugins = {
  fzf-tab = {
    name = "fzf-tab";
    src = "${pkgs.zsh-fzf-tab}/share/fzf-tab";
  };
  zsh-autosuggestions = {
    name = "zsh-autosuggestions";
    src = "${pkgs.zsh-autosuggestions}/share/zsh-autosuggestions";
    file = "zsh-autosuggestions.zsh";
  };
  zsh-syntax-highlighting = {
    name = "zsh-syntax-highlighting";
    src = "${pkgs.zsh-syntax-highlighting}/share/zsh-syntax-highlighting";
    file = "zsh-syntax-highlighting.zsh";
  };
};