Make Material2d pipeline systems public (#8642)

# Objective

Make `Material2dPipeline` reusable. This was already done for PBR
materials in #7548.

## Solution

Expose `extract_materials_2d`, `prepare_materials_2d` and
`ExtractedMaterials2d`.

---

## Changelog

- bevy_sprite: Make `prepare_materials_2d`, `extract_materials_2d` and
`ExtractedMaterials2d` public.

crates/bevy_sprite/src/mesh2d/material.rs

@@ -432,7 +432,7 @@ pub struct PreparedMaterial2d<T: Material2d> {
 }
 
 #[derive(Resource)]
-struct ExtractedMaterials2d<M: Material2d> {
+pub struct ExtractedMaterials2d<M: Material2d> {
  extracted: Vec<(Handle<M>, M)>,
  removed: Vec<Handle<M>>,
 }
@@ -458,7 +458,7 @@ impl<T: Material2d> Default for RenderMaterials2d<T> {
 
 /// This system extracts all created or modified assets of the corresponding [`Material2d`] type
 /// into the "render world".
-fn extract_materials_2d<M: Material2d>(
+pub fn extract_materials_2d<M: Material2d>(
  mut commands: Commands,
  mut events: Extract<EventReader<AssetEvent<M>>>,
  assets: Extract<Res<Assets<M>>>,
@@ -505,7 +505,7 @@ impl<M: Material2d> Default for PrepareNextFrameMaterials<M> {
 
 /// This system prepares all assets of the corresponding [`Material2d`] type
 /// which where extracted this frame for the GPU.
-fn prepare_materials_2d<M: Material2d>(
+pub fn prepare_materials_2d<M: Material2d>(
  mut prepare_next_frame: Local<PrepareNextFrameMaterials<M>>,
  mut extracted_assets: ResMut<ExtractedMaterials2d<M>>,
  mut render_materials: ResMut<RenderMaterials2d<M>>,