Custom shader support

[tattn]

Summary

Adds custom material shader support to VRMRealityKit.

Changes

• Added a material shader chain to GLTFEntityLoader and VRMEntityLoader.
• Refactored MToon rendering to use the new shader architecture.
• Added support for custom shaders and MToon conversion for standard glTF materials.
• Generalized animatable material state and additional render passes.
• Updated examples, documentation, and tests for the new API.

Breaking Changes

The previous isMToonEnabled / isOutlineEnabled loader options are replaced by the new shaders configuration.