Update OMI_physics_joint to be more like KHR physics joints

[aaronfranke]

This PR updates the Godot implementation of OMI_physics_joint in this repo to match omigroup/gltf-extensions#232

These changes will not work in Godot 4.3. The following PRs are required on Godot's side:

• GLTF: Add append_gltf_node to GLTFState godotengine/godot#96468
• GLTF: Add import_pre_generate and export_post_convert extension steps godotengine/godot#96465
• Not required but nice to have: Generic6DOFJoint3D: Rename FLAG_ENABLE_MOTOR to FLAG_ENABLE_ANGULAR_MOTOR godotengine/godot#96458
• Not required but nice to have: Add units and docs for Joint3D/2D stiffness and damping godotengine/godot#96523

The ability to load old OMI_physics_joint files is preserved by this PR. Both the new and old code co-exist when importing, but when exporting, it only exports in the new format.

See the gltf-extensions repo PR for more information: omigroup/gltf-extensions#232