readline: replace _questionCancel with a symbol

This commit avoids exposing a new underscored property on
readline Interface instances.

PR-URL: #37094
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/readline.js

@@ -117,6 +117,7 @@ const kMincrlfDelay = 100;
 const lineEnding = /\r?\n|\r(?!\n)/;
 
 const kLineObjectStream = Symbol('line object stream');
+const kQuestionCancel = Symbol('kQuestionCancel');
 
 const KEYPRESS_DECODER = Symbol('keypress-decoder');
 const ESCAPE_DECODER = Symbol('escape-decoder');
@@ -225,7 +226,7 @@ function Interface(input, output, completer, terminal) {
       };
   }
 
-  this._questionCancel = FunctionPrototypeBind(_questionCancel, this);
+  this[kQuestionCancel] = FunctionPrototypeBind(_questionCancel, this);
 
   this.setPrompt(prompt);
 
@@ -376,7 +377,7 @@ Interface.prototype.question = function(query, options, cb) {
 
   if (options.signal) {
     options.signal.addEventListener('abort', () => {
-      this._questionCancel();
+      this[kQuestionCancel]();
     }, { once: true });
   }