Migrate VRMConstraint to BoneConstraint3D

[fire]

Useful for testing.

[fire]

Settings from tokage.

[TokageItLab]

Note also that RetargetModifier must be used to preserve the rest
godotengine/godot#97824

[fire]

Paraphrased from Tokage

If we import VRM into Godot, the behaviour will be covered. However, if we export VRM from Godot, I believe we need some validation to alert us if the user is using BoneConstraint3D for features that VRMConstraint does not support.

[fire]

From Tokage. Need to check when export to VRM Constraint:

• CopyTransformModifier -> RotationConstraint
  • Copy
    • Must be enabled Rotation only
  • Axes
    • Must be enabled on all axes
  • Invert
    • Must be disabled on all axes
  • Relative/Additive
    • Should be enabled, but depends on SceneTree order (so SceneTree order should be validated separately)
• ConvertTransformModifier -> RollConstraint
  • Transform Mode
    • Must be Rotation
  • Range Min
    • Must be -180deg
  • Range Max
    • Must be 180deg
  • Use Euler
    • Must be disabled
  • Relative/Additive
    • Should be enabled, but depends on SceneTree order (so SceneTree order should be validated separately)
• TrackBoneModifier -> AimConstraint
  • Use Euler
    • Must be disabled