feat(body_mesh): Add body mesh separator module with edge dissolution

- Add body_mesh submodule with VRM_OT_separate_non_skin_faces operator
- Add VRM_PT_body_mesh_separator panel for UI integration
- Add utility functions for mesh operations and edge dissolution logic
- Add comprehensive edge dissolution configuration files for all body parts (head, torso, arms, legs, hands) with UV coordinates
- Add test utilities for validating mesh separation operations
- Add .gitignore to exclude Python cache files and test artifacts
- Register body_mesh module in editor package initialization
- Update registration.py to include new body mesh components

editor/body_mesh/dissolve_edges_arms.json

@@ -0,0 +1,31 @@
+{
+  "description": "VRM模型手臂区域需要融并的边索引列表（用于三角面转四边面）",
+  "region": "arms",
+  "total_edges": 834,
+  "edge_indices": [
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+}