feat: Add new Geometry Nodes groups for image relief and shadeless shader

- Implemented the "O Image Relief Plane" Geometry Nodes group to create a fixed-base image relief using depth images and various parameters.
- Created the "O Shadeless" shader node group to display color without scene lighting while preserving shadows and reflections.
- Introduced a preview feature for node groups, allowing for interactive visualization of node arrangements in an HTML format.
- Added a template HTML file for rendering node previews with customizable views and controls.

tests/conftest.py

@@ -0,0 +1,5 @@
+"""Expose project source packages to the test suite."""
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).resolve().parents[1] / "src"))