vars: add .exists to files

nixosModules/clanCore/vars/interface.nix

@@ -304,6 +304,15 @@ in
                       description = "The unix file mode of the file. Must be a 4-digit octal number.";
                       default = "0400";
                     };
+                    exists = mkOption {
+                      description = ''
+                        Returns true if the file exists, This is used to guard against reading not set value in evaluation.
+                        This currently only works for non secret files.
+                      '';
+                      type = bool;
+                      default = if file.config.secret then throw "Cannot determine existance of secret file" else false;
+                      defaultText = "Throws error because the existance of a secret file cannot be determined";
+                    };
                     value =
                       mkOption {
                         description = ''