Geometry Nodes: add Inspection Index to Repeat Zone

Previously, it was only possible to inspect the data from the first iteration. That
applied to both, the viewer node as well as socket inspection. Now, there is a
new `Inspection Index` setting in the zone properties. It specifies which iteration
should be used by the inspection features.

In theory we could support features like counting the index from the end, but
that can be done separately as well, as it likely requires more UI.

Pull Request: https://projects.blender.org/blender/blender/pulls/112818

scripts/startup/bl_ui/space_node.py

@@ -1140,6 +1140,8 @@ class NODE_PT_repeat_zone_items(Panel):
             layout.use_property_decorate = False
             layout.prop(active_item, "socket_type")
 
+        layout.prop(output_node, "inspection_index")
+
 
 # Grease Pencil properties
 class NODE_PT_annotation(AnnotationDataPanel, Panel):