Contrary to `CompositorNodeCustomGroup` or `ShaderNodeCustomGroup`,
`GeometryNodeCustomGroups` have to define their own poll function.
This is because their is no predefined poll function for `GeometryNode`,
and it may not be clear for addon developers why `GeometryNode` would
be special here.
This adds `GeometryNode` to `bpy_types.py` and defines such a function
for it like for other builtin node types.
Differential Revision: https://developer.blender.org/D14775
2062116924