Describe the project you are working on

Making a level for a 3D Top Down shooter: https://github.com/Nodragem/twin-stick-shooter-starter-kit

Describe the problem or limitation you are having in your project

In 4.3dev6, if I drag and drop a 3D asset in the 3D Viewport it is added as a child and automatically selected; which is not necessarily useful.

Thanks to this PR: godotengine/godot#87623, we've got the following behaviour:

By default, drag-and-dropping adds the scene as a child of the selected node. Hold Alt when releasing the mouse to add the scene as a child of the root node, or Shift to add the scene as a sibling of the selected node.

And thanks to this PR: godotengine/godot#84347, the latest added node is automativally selected.

As discussed here #9613, this accidentially creates the following behaviours:

• default: add new node as child of selected node and select automatically the new node;
• ALT modifier: add new node as child of root and select automatically the new node;
• SHIFT modifier: add new node as sibbling of currently selected node and select automatically the new node.

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

These accidental features are very exciting for level designers. But I would like to propose that we swap the SHIFT and default behaviour. Indeed, when editing a level, to add as sibling and select automatically the new node is what you want 90% of the time.

Alternatively, or in addition, the default, ALT and SHIFT behaviours could be changed in the Editor Settings.

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

Just need to a few variables in the code.
Then for having it in the Editor Settings, it might me more involved.

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

Nope, it is hard coded.

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

This is a small change on an existing feature.