Animate the radio widget

[bradysimon]

Overview

This PR uses the new animation API to animate the radio widget. The current animation scales/fades the dot icon as the is_selected state changes.

This also gates the animation behind a new animations feature that Hector specified and adds an optional animations feature to the scrollable example to demonstrate this change. To test this change locally:

• with animations: cargo run -p scrollable --features animations
• without animations: cargo run -p scrollable

Preview

Here's what the animation looks like (in light mode since the radio in dark mode isn't as clear):

animated-radio.mov

Side note: Hector mentioned on Discord to avoid animating more complex types like colors/backgrounds, so this PR doesn't add any general Style animation. Only f32/bool animations for now.

[bradysimon]

An alternative animation would be to make the dot scale from a size of zero to its final size. I think I prefer the current animation but I want to at least present both options:

animated-radio-alt.mov