VisualScriptEditor and VisualScriptCustomNodes after namespaces

### Describe the project you are working on

General VisualScript Improvements

### Describe the problem or limitation you are having in your project

After https://github.com/godotengine/godot/pull/52023
There now are 2 singletons for accessing VisualScriptEditor.

I think the name 'VisualScriptCustomNodes' is misleading as you want to reference the editor not a list of nodes or a specific script.

### Describe the feature / enhancement and how it helps to overcome the problem or limitation

Remove Class VisualScriptCustomNodes

### Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Move the exposed add_custom_node function to VisualScriptEditor.
Move the exposed remove_custom_node function to VisualScriptEditor.
Move the private add_custom_node function to VisualScriptEditor.
Remove VisualScriptEditor
Keep VisualScriptEditor singleton exposed

### If this enhancement will not be used often, can it be worked around with a few lines of script?

Enhancement for clarity and ease of maintenance.

### Is there a reason why this should be core and not an add-on in the asset library?

Class VisualScriptCustomNodes is cleaning up the core excess

CC: @mhilbrunner