Add 'Smoothing Groups' option for exporting polygon smoothing groups. Generates 'bitflags' smoothgroups values, ensuring no faces from different groups sharing a same boundary edge or vertex have the same bitflag value. NOTE: This type of smoothgroups will run out of group values (aka bitflags) with some specific topologies, e.g. a fan of over 31 flat faces sharing a same vertex (like the sides of a cone). In such cases, once all available values have been used, the remaining faces will get `0` (aka 'no group') value assigned. Original issue https://projects.blender.org/blender/blender-addons/issues/104434 Pull Request: https://projects.blender.org/blender/blender/pulls/135646