Move fade animation to the Animate component

[kjellr]

Once #17106 is merged in, this PR will handle the final migration of animations from _animations.scss into the Animate component. Moving these into the Animate component will allow the animations to be much more self-contained, and prevent us from separately declaring the keyframe declarations in each environment where they need to be used.

Some of these are a bit beyond my expertise, so if anyone else wants to pitch in, please feel free to push directly to this PR (and thank you in advance! 🙌)!

Tasks

• Add a new fade-in animation to the Animate component
• Update block-movers to use the new animation
• Update inserter-with-shortcuts to use the new animation
• Update inserter help panel to use the new animation
• Update modal to use the new animation
• Update fullscreen-mode to use the new animation
• Delete the _animations.scss file, and stop it from being included in all other SCSS files.
• Remove keyframes from packages/edit-widgets/src/style.scss, playground/src/style.scss, and packages/edit-post/src/style.scss
• Update documentation

(If it ends up making sense to break these out into separate PRs, we can do that)

[annezazu]

Closing this out due to inactivity!