Alias world_to_viewport for easier migration (#5298)

# Objective

When someone searches in rustdoc for `world_to_screen`, they now will
find `world_to_viewport`. The method was renamed in 0.8, it would be
nice to allow users to find the new name more easily.

---

crates/bevy_render/src/camera/camera.rs

@@ -189,6 +189,7 @@ impl Camera {
     ///
     /// To get the coordinates in Normalized Device Coordinates, you should use
     /// [`world_to_ndc`](Self::world_to_ndc).
+    #[doc(alias = "world_to_screen")]
     pub fn world_to_viewport(
         &self,
         camera_transform: &GlobalTransform,