feat(external API): add feedbackPromptDisplayed event

Adds a new event fired when Jitsi Meet shows the feedback prompt.

modules/API/API.js

@@ -541,6 +541,16 @@ class API {
         this._sendEvent({ name: 'feedback-submitted' });
     }
 
+    /**
+     * Notify external application (if API is enabled) that the feedback prompt
+     * has been displayed.
+     *
+     * @returns {void}
+     */
+    notifyFeedbackPromptDisplayed() {
+        this._sendEvent({ name: 'feedback-prompt-displayed' });
+    }
+
     /**
      * Notify external application (if API is enabled) that the screen sharing
      * has been turned on/off.