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/__init__.py

@@ -1 +1,2 @@
 # SPDX-License-Identifier: MIT OR GPL-3.0-or-later
+from . import body_mesh