addSlot(intent, entity, mandatory = false, questions) {
    this.isEmpty = false;
    if (!this.intents[intent]) {
      this.intents[intent] = {};
    }
    this.intents[intent][entity] = {
      intent,
      entity,
      mandatory,
      locales: questions || {},
    };
    return this.intents[intent][entity];
  }
Adds a new slot for a given intent and entity.
@param {String} intent Name of the intent.
@param {String} entity Name of the entity.
@param {boolean} [mandatory=false] Flag indicating if is mandatory or optional.
@param {Object} [questions] Question to ask when is mandatory, by locale.
@returns {Object} New slot instance.