`NOTIFICATION_POSTINITIALIZE` doesn't happen when an object is reloaded

### Godot version

Godot 4.5.1

### godot-cpp version

godot-cpp 4.5

### System information

Godot v4.5.1.stable - Pop!_OS 22.04 LTS on X11 - X11 display driver, Single-window, 3 monitors - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 4070 Ti - AMD Ryzen 9 7950X 16-Core Processor (32 threads) - 61.94 GiB memory

### Issue description

Currently, it's `memnew()` that calls `notification(NOTIFICATION_POSTINITIALIZE)` on the whole Godot object, which will eventually trickle down to any extension classes that are attached to it.

However, if an object is reloaded, `memnew()` doesn't happen. In part, this is good, because we don't want the whole Godot object to get `NOTIFICATION_POSTINITIALIZE` again.

But I think we _do_ want for the instance binding on the godot-cpp side to get `NOTIFICATION_POSTINITIALIZE` again, because some extension classes may not work right if it doesn't happen.

So, I suspect this is a bug, but I haven't looked into too much yet

(It was @raulsntos who pointed this out to me)

### Steps to reproduce

1. Make a `Node` in an extension that does something on `NOTIFICATION_POSTINITIALIZE`
2. Add this node to a scene in the editor
3. Rebuild the extension to initiate reload
4. Notice that it didn't do it's thing on `NOTIFICATION_POSTINITIALIZE`

### Minimal reproduction project

Not yet :-)