Improve internal composable API for className

[itsdouges]

We can't easily compose things from className and keyframes ATM. Since this library wants to try and do things via CSS animation (over JS animation) we need to come up with a solution.

API thoughts:

setChildProps({
  // composable style
  style: prevStyle => ({ ...prevStyle }),

  // composable class via objects
  classNameStyle: prevStyle => ({ ...prevStyle }),

  // composable keyframes
  keyframes: prevKeyframes => compose => compose(prevKeyframes, newKeyframes, 'animation-name'),
});

• Removes className from API (deprecated first)
• Adds classNameStyle to API
• Remove emotion dependency

[itsdouges]

Keyframes is done.