refactor(systemModules:shell): move shell module to reflect changes to how user shell is defined

system/modules/shell/zsh/default.nix

@@ -0,0 +1,25 @@
+{ lib, config, ... }:
+
+let
+  inherit (lib) mkIf mkEnableOption;
+  adminShell = config.systemModules.host.admin.shell;
+  cfg = config.systemModules.shell.zsh;
+in
+
+{
+
+  options.systemModules.zsh.enable = mkEnableOption "Enable zsh module";
+
+  config = mkIf (adminShell == "zsh" || cfg.enable) {
+    programs.zsh = {
+      enable = true;
+      enableCompletion = true;
+      syntaxHighlighting.enable = true;
+      autosuggestions = {
+        enable = true;
+        async = true;
+      };
+    };
+    environment.pathsToLink = ["/share/zsh"];
+  };
+}