feat(node_assets): add image plane geometry nodes and validation scripts

- Implemented `image_plane.py` to build the Image Plane Geometry Nodes group with customizable parameters for geometry, subdivide, and thickness.
- Created `image_plane_validation.py` to validate the functionality of the image plane, ensuring correct geometry generation and material assignment.
- Developed `material_layers.py` to construct shader node groups for image materials, including depth and normal handling.
- Authored `plane_layouts.py` to define node arrangements for image geometry assets.
- Introduced `shadeless.py` to create a shadeless shader node group that displays color without scene lighting while preserving shadows and reflections.

src/anyimage/menu.py

@@ -16,7 +16,7 @@ class AnyImageImageMenu(bpy.types.Menu):
         layout.operator(
             "anyimage.convert_to_plane",
             text="Convert to Plane",
-            icon="MESH_PLANE",
+            icon="IMAGE_PLANE",
         )
         layout.operator(
             "anyimage.convert_to_depth_plane",