Don't require going through bevy_animation::prelude to get to certain items in bevy_animation (#14979)

# Objective
* Fixes https://github.com/bevyengine/bevy/issues/14889

## Solution
Exposes `bevy_animation::{animatable, graph, transition}` to the world.

## Testing
- Did you test these changes? If so, how?
- These changes do not need testing, as they do not modify/add/remove
any functionality.
- ~~Are there any parts that need more testing?~~
- ~~How can other people (reviewers) test your changes? Is there
anything specific they need to know?~~
- ~~If relevant, what platforms did you test these changes on, and are
there any important ones you can't test?~~

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

crates/bevy_animation/src/animatable.rs

@@ -1,3 +1,5 @@
+//! Traits and type for interpolating between values.
+
 use crate::util;
 use bevy_color::{Laba, LinearRgba, Oklaba, Srgba, Xyza};
 use bevy_ecs::world::World;

crates/bevy_animation/src/lib.rs

@@ -7,9 +7,9 @@
 
 //! Animation for the game engine Bevy
 
-mod animatable;
-mod graph;
-mod transition;
+pub mod animatable;
+pub mod graph;
+pub mod transition;
 mod util;
 
 use std::cell::RefCell;