fix 2d examples not compiling

[spookyvision]

Objective

• fix 2d examples not compiling, error was:

$ cargo run --example $ANY_AVIAN2D_EXAMPLE
error[E0433]: failed to resolve: use of undeclared type `Animatable`
   --> crates/avian2d/../../src/diagnostics/ui.rs:434:24
    |
434 |         text_color.0 = Animatable::interpolate(&green, &red, t.min(1.0)).into();
    |                        ^^^^^^^^^^ use of undeclared type `Animatable`

Solution

• add bevy_animation to bevy feature flags in examples_common_2d

[Jondolf]

Hi, thanks and sorry for getting to this so late!

There's still a few issues here: I believe this only fixes the problem for the 2D examples, not 3D or for Avian itself. If someone was using Avian without having bevy_animation enabled for Bevy, they could get the same problem.

I've fixed this in #711 by just getting rid of the use of Animatable altogether.

[spookyvision]

I think the reason I specifically fixed this only for 2D was that the 3D examples were building fine (probably because they didn't use that UI overlay), but I guess I was missing the forest for the trees here 😅