Subscriptions: add the possibility to subscribe only to replies to one's comment

Currently, when adding a comment to a site, you can subscribe to the site or to the follow-up comments. If you do subscribe to the comments, you'll receive notifications every time someone adds a new comment to that post.

It would be great to let the users subscribe only to the replies to their comments. That way they could keep an eye on replies but won't get hundreds of notifications for other, irrelevant ones.

A user reported it here:
https://wordpress.org/support/topic/subscribe-to-new-comments-not-working/#post-13179495