Geometry Nodes: Add Combine and Separate XYZ nodes for attributes

These are similar to the regular "Combine XYZ" and "Separate XYZ" nodes,
but they work on attributes. They will make it easier to switch between
vector attributes and float attributes.

Differential Revision: https://developer.blender.org/D10308

release/scripts/startup/nodeitems_builtins.py

@@ -491,6 +491,8 @@ geometry_node_categories = [
         NodeItem("GeometryNodeAttributeColorRamp"),
         NodeItem("GeometryNodeAttributeVectorMath"),
         NodeItem("GeometryNodeAttributeSampleTexture"),
+        NodeItem("GeometryNodeAttributeCombineXYZ"),
+        NodeItem("GeometryNodeAttributeSeparateXYZ"),
     ]),
     GeometryNodeCategory("GEO_COLOR", "Color", items=[
         NodeItem("ShaderNodeValToRGB"),