Implement duplication of Alt-dragged layers within the Layers panel

[sneaxhuh]

This PR introduces the ability to duplicate selected layers within the editor. Users can now duplicate layers by dragging them while holding the Alt key.

This includes:

Adding a new DuplicateSelectedLayers message.

Implementing the logic to copy and insert duplicated layers into the document.

Updating the frontend to detect the Alt key during drag operations and trigger the duplication.

Exposing the duplicateLayer function in the WASM API.

[sneaxhuh]

Hi @Keavon , can you please review this. Thanks

[Keavon]

Will do, no promises on when though. Still backlogged on code reviews.

[sneaxhuh]

@Keavon , just a gentle reminder

[Keavon]

Any updates? Just a reminder.