Unify linear interpolation traits

In Bevy, there are two traits with methods for linear interpolation:

* [`bevy_aniumation::animatable::Animatable`](https://docs.rs/bevy_animation/latest/bevy_animation/animatable/trait.Animatable.html)
* [`bevy_math::common_traints::StableInterpolate`](https://docs.rs/bevy_math/latest/bevy_math/common_traits/trait.StableInterpolate.html)

The former has an `interpolate` method, and the latter has `interpolate_stable` method, with the same signature. They also have different requirements (former wants `Reflect` and also `fn blend` implementation, latter needs it to be *stable* interpolation).

It might be a good idea to have a single general "interpolatable" trait, either unifying all of these properties into one, or as a supertrait of both.

(I apologize for using the non-fancy blank issue template; none of the others felt like they fit, since this isn't really a feature request nor a bug with Bevy)

Read the Animatable trait in bevy_animation::animatable and StableInterpolate in bevy_math::common_traits first, comparing their methods and requirements. Decide whether a unified trait or a supertrait best fits Bevy's API, then confirm the relevant tests and documentation cover the chosen design.

Written by the indexing model from the issue text.