UI: Add Wavelength unit for scene-independent nanometer-scale inputs

Currently this is only used for the Wavelength node, but it will also be relevant for the Thin Film Interference support in the Principled BSDF.

Pull Request: https://projects.blender.org/blender/blender/pulls/120900

source/blender/python/intern/bpy_utils_units.cc

@@ -52,6 +52,7 @@ static const char *bpyunits_ucategories_items[] = {
     "CAMERA",
     "POWER",
     "TEMPERATURE",
+    "WAVELENGTH",
     nullptr,
 };