Add glTF scene conversion

[greeble-dev]

An alternative to bevyengine#20394 that takes a more conservative approach and avoids removing existing code.

Comparison:

This PR

• Replaces use_model_forward_direction: bool with convert_coordinates: GltfConvertCoordinates
• GltfConvertCoordinates has mutually exclusive Nodes and Scenes options.
• Nodes is same as 0.17
• Scenes is new and just transforms the scene root.

struct GltfPlugin {
-    use_model_forward_direction: bool,
+    convert_coordinates: GltfConvertCoordinates,
 }

+enum GltfConvertCoordinates {
+    #[default]
+    Off,
+    Scenes,
+    Nodes,
+}

bevyengine#20394

• Removes node conversion entirely.
• Adds a scene conversion option (same as this PR).