[fix] migrate to blender 3.6 LTS

- change bl_info["support"] to COMMUNITY. because Blender do not support TESTINg anymore.
- now plugin should be installed in addons folder, not addons_contrib, due to blender changes.
- remove the reference about mesh.polypons.loop_total. because it is readonly now. (blender 3.6 changed)
- change uv assign method. use new properties instead. (MeshUVLoop is deprecated in blender 3.5 and removed in blender 4.0)

ballance_blender_plugin/MODS_flatten_uv.py

@@ -110,7 +110,7 @@ def _real_flatten_uv(mesh, reference_edge, scale_data: ScaleDataUnion):
         mesh.uv_layers.new(do_init=False)
 
     bm = bmesh.from_edit_mesh(mesh)
-    uv_lay = bm.loops.layers.uv.active
+    uv_lay = bm.loops.layers.uv.active # NOTE: this is a part of bmesh. not affected by Blender 3.5 CHANGED.
     for face in bm.faces:
         # ========== only process selected face ==========
         if not face.select: