impl Reflect and Debug for Mesh2dHandle (bevyengine#4368)

# Objective

An entity spawned with `MaterialMesh2dBundle<M>` cannot be saved and spawned using `DynamicScene` because the `Mesh2dHandle` component does not `impl Reflect`.  

## Solution

Add `#[derive(Reflect)]` and `#[reflect(Component)]` to `Mesh2dHandle`, and call `register_type` in `SpritePlugin`.  Also add `#[derive(Debug)]` since I'm touching the `derive`s anyway.

Author: chescock

crates/bevy_sprite/src/lib.rs

@@ -57,6 +57,7 @@ impl Plugin for SpritePlugin {
         shaders.set_untracked(SPRITE_SHADER_HANDLE, sprite_shader);
         app.add_asset::<TextureAtlas>()
             .register_type::<Sprite>()
+            .register_type::<Mesh2dHandle>()
             .add_plugin(Mesh2dRenderPlugin)
             .add_plugin(ColorMaterialPlugin);
 

crates/bevy_sprite/src/mesh2d/mesh.rs

@@ -5,7 +5,7 @@ use bevy_ecs::{
     system::{lifetimeless::*, SystemParamItem},
 };
 use bevy_math::{Mat4, Size};
-use bevy_reflect::TypeUuid;
+use bevy_reflect::{Reflect, TypeUuid};
 use bevy_render::{
     mesh::{GpuBufferInfo, Mesh, MeshVertexBufferLayout},
     render_asset::RenderAssets,
@@ -22,7 +22,8 @@ use bevy_transform::components::GlobalTransform;
 /// Component for rendering with meshes in the 2d pipeline, usually with a [2d material](crate::Material2d) such as [`ColorMaterial`](crate::ColorMaterial).
 ///
 /// It wraps a [`Handle<Mesh>`] to differentiate from the 3d pipelines which use the handles directly as components
-#[derive(Default, Clone, Component)]
+#[derive(Default, Clone, Component, Debug, Reflect)]
+#[reflect(Component)]
 pub struct Mesh2dHandle(pub Handle<Mesh>);
 
 impl From<Handle<Mesh>> for Mesh2dHandle {