refactor!: Replace Offset with opacityFrom and opacityTo in ColorEffect (#2876)

`ColorEffect`'s API was a bit confusing because it used `Offset` for
getting values of opacity start and end from user. This PR changes that
to use optional `opacityFrom` and `opacityTo` double parameters. It also
adds validate checks on these value to make sure that are between 0 and
1.

examples/lib/stories/collision_detection/multiple_worlds_example.dart

@@ -69,11 +69,11 @@ class CollidableEmber extends Ember with CollisionCallbacks {
     add(
       ColorEffect(
         index < 2 ? Colors.red : Colors.green,
-        const Offset(0, 0.9),
         EffectController(
           duration: 0.2,
           alternate: true,
         ),
+        opacityTo: 0.9,
       ),
     );
   }