EditorToaster does not expose `_popup_str` anymore, regression in 4.3.dev3

### Tested versions

v4.3.dev3.official [36e943b6b]
v4.3.dev4.official [df78c0636]
v4.3.dev5.official [89f70e98d]

### System information

Godot v4.3.dev5 - Windows 10.0.22631 - Vulkan (Forward+) - dedicated NVIDIA GeForce GTX 1080 Ti (NVIDIA; 31.0.15.5152) - AMD Ryzen 9 7900X 12-Core Processor (24 Threads)

### Issue description

I try to use the `EditorToaster` from the editor by getting the node and `_editor_toaster.call("_popup_str", message, severity, tooltip)`. Since `v4.3.dev3.official [36e943b6b]` the method is not callable anymore.

The last time it worked was in `v4.3.dev2.official [352434668]`.

### Steps to reproduce

Open the MRP and enable/disable the plugin, which will trigger the toster.

### Minimal reproduction project (MRP)

[toaster_example.zip](https://github.com/godotengine/godot/files/14998120/toaster_example.zip)

Start at the EditorToaster implementation and inspect how _popup_str is exposed or called; use toaster_example.zip to reproduce the problem by enabling and disabling the plugin. Done when the regression between 4.3.dev2 and 4.3.dev3 is addressed and the reproduction works again.

Written by the indexing model from the issue text.