Add a node compatibility handler for ProximityGroup

This converts ProximityGroup nodes from 3.x projects into Node3Ds
(their parent), rather than bare Nodes.

scene/register_scene_types.cpp

@@ -960,6 +960,7 @@ void register_scene_types() {
 	ClassDB::add_compatibility_class("Navigation3D", "Node3D");
 	ClassDB::add_compatibility_class("Navigation2D", "Node2D");
 	ClassDB::add_compatibility_class("OpenSimplexNoise", "FastNoiseLite");
+	ClassDB::add_compatibility_class("ProximityGroup", "Node3D");
 	ClassDB::add_compatibility_class("ToolButton", "Button");
 	ClassDB::add_compatibility_class("YSort", "Node2D");
 	// Portal and room occlusion was replaced by raster occlusion (OccluderInstance3D node).