zsh: add enableSyntaxHighlighting option

Add a simple way to enable syntax highlighting for zsh using https://github.com/zsh-users/zsh-syntax-highlighting.

modules/programs/zsh.nix

@@ -290,6 +290,11 @@ in
         description = "Enable zsh autosuggestions";
       };
 
+      enableSyntaxHighlighting = mkOption {
+        default = false;
+        description = "Enable zsh syntax highlighting";
+      };
+
       history = mkOption {
         type = historyModule;
         default = {};
@@ -482,6 +487,10 @@ in
           "source ${pkgs.zsh-autosuggestions}/share/zsh-autosuggestions/zsh-autosuggestions.zsh"
         }
 
+        ${optionalString cfg.enableSyntaxHighlighting
+          "source ${pkgs.zsh-syntax-highlighting}/share/zsh-syntax-highlighting/zsh-syntax-highlighting.zsh"
+        }
+
         # Environment variables
         . "${config.home.profileDirectory}/etc/profile.d/hm-session-vars.sh"
         ${envVarsStr}