Scope keyframe names defined in <style scoped>

[yhatt]

When defined @keyframes in <style scoped>, Marpit will scope the keyframe name, and replace the keyframe name defined in animation and animation-name declarations within <style scoped> into scoped keyframe.

It's helpful for defining slide animations without thinking about confliction with global animation keyframes.

<style scoped>
@keyframes local-animation {
  from { transform: translateX(100%); }
  to { transform: none; }
}
h1 {
  animation: 1s ease-out local-animation;
}
</style>

The above definition will transform into this:

@keyframes local-animation-RVSdalDm {
  from { transform: translateX(100%); }
  to { transform: none; }
}

div.marpit > section[data-marpit-scope-RVSdalDm] h1 {
  animation: 1s ease-out local-animation-RVSdalDm;
}

Marpit will replace only animation names defined by <style scoped> in the same slide. So global animation keyframes are still working.

Resolves #231.