Organized scene_viewer into plugins for reuse and organization (#6936)

# Objective

This PR reorganizes majority of the scene viewer example into a module of plugins which then allows reuse of functionality among new or existing examples.  In addition, this enables the scene viewer to be more succinct and showcase the distinct cases of camera control and scene control.

This work is to support future work in organization and future examples.  A more complicated 3D scene example has been requested by the community (#6551) which requests functionality currently included in scene_viewer, but previously inaccessible.  The future example can now just utilize the two plugins created here.  The existing example [animated_fox example] can utilize the scene creation and animation control functionality of `SceneViewerPlugin`.

## Solution

- Created a `scene_viewer` module inside the `tools` example folder.
- Created two plugins:  `SceneViewerPlugin` (gltf scene loading, animation control, camera tracking control, light control) and `CameraControllerPlugin` (controllable camera).
- Original `scene_viewer.rs` moved to `scene_viewer/main.rs` and now utilizes the two plugins.

Cargo.toml

@@ -1358,7 +1358,7 @@ wasm = true
 # Tools
 [[example]]
 name = "scene_viewer"
-path = "examples/tools/scene_viewer.rs"
+path = "examples/tools/scene_viewer/main.rs"
 
 [package.metadata.example.scene_viewer]
 name = "Scene Viewer"

examples/README.md

@@ -295,7 +295,7 @@ Example | Description
 Example | Description
 --- | ---
 [Gamepad Viewer](../examples/tools/gamepad_viewer.rs) | Shows a visualization of gamepad buttons, sticks, and triggers
-[Scene Viewer](../examples/tools/scene_viewer.rs) | A simple way to view glTF models with Bevy. Just run `cargo run --release --example scene_viewer /path/to/model.gltf#Scene0`, replacing the path as appropriate. With no arguments it will load the FieldHelmet glTF model from the repository assets subdirectory
+[Scene Viewer](../examples/tools/scene_viewer/main.rs) | A simple way to view glTF models with Bevy. Just run `cargo run --release --example scene_viewer /path/to/model.gltf#Scene0`, replacing the path as appropriate. With no arguments it will load the FieldHelmet glTF model from the repository assets subdirectory
 
 ## Transforms