Make NOTIFICATION_MOVED_IN_PARENT opt-in.

[Zylann]

Makes it faster to remove non-2D nodes from the scene tree when they have lots of siblings.

This partly solves #61929 for non-2D nodes, with a very simple change. This is not exclusive to possible improvements mentionned in #61929 (comment) (although so far they are for Godot 3).
Only 2D nodes use this notification so I thought it made sense if that notification wasn't always sent to others in the first place. It was causing stalls in my 3D game.

[lawnjelly]

This is the bitflags approach I was referring to in #65581 (comment) . (Except using a single bool because only supporting one notification type).

To quote from my post there:

Another idea is for nodes to register their interest in a notification type (via a set of bitflags), and check this prior to sending the more heavyweight notification.

Although it's an improvement on the before case, there are a couple of problems:

• It is compat breaking, as users have to register an interest (if they e.g. create new nodes in a module that want to respond to particular notifications etc)
• It doesn't prevent the notification being sent multiple times to nodes that are interested.

That said, registering for interest is a worthy option that can be used in addition to other approaches e.g. #65581 and #62444.