feat: Add VRM Blender addon with complete import/export functionality

- Add core VRM addon infrastructure with manifest and registration
- Add common utilities module with file system, logging, and conversion helpers
- Add human bone mapper with support for multiple rigging standards (Mixamo, MMD, Unreal, Rigify, etc.)
- Add VRM 0.x and 1.x format support with property groups and handlers
- Add editor UI panels for VRM metadata, spring bones, and MToon materials
- Add exporter with glTF2 extension support for VRM format serialization
- Add importer with scene reconstruction and armature generation
- Add MToon shader support with auto-setup and material migration
- Add spring bone physics simulation with constraint handling
- Add node constraint editor for advanced rigging control
- Add comprehensive validation and error handling with user dialogs
- Add scene watcher for real-time property synchronization
- Add workspace management and preference system
- Include Python cache files and Blender manifest configuration
- This is the initial commit establishing the complete VRM addon ecosystem for Blender

common/legacy_gltf.py

@@ -0,0 +1,16 @@
+# SPDX-License-Identifier: MIT OR GPL-3.0-or-later
+"""Legacy glTF shader that existed before version 2.10.5.
+
+https://github.com/saturday06/VRM-Addon-for-Blender/tree/2_10_5
+"""
+
+from typing import Final
+
+TEXTURE_INPUT_NAMES: Final = (
+    "color_texture",
+    "normal",
+    "emissive_texture",
+    "occlusion_texture",
+)
+VAL_INPUT_NAMES: Final = ("metallic", "roughness", "unlit")
+RGBA_INPUT_NAMES: Final = ("base_Color", "emissive_color")