Move most animation processing to script

This is preparation for sharing this code with layout_2020 and
implementing selective off-the-main-thread animations.

We still look for nodes not in the flow tree in the layout thread.

components/script/lib.rs

@@ -48,6 +48,7 @@ extern crate servo_atoms;
 extern crate style;
 
 mod animation_timeline;
+mod animations;
 #[warn(deprecated)]
 #[macro_use]
 mod task;