add css support for animation-fill-mode

related to https://github.com/Volumetrics-io/mrjs/pull/600

#600 added javascript ability to do so following threejs's paradigm of `clampWhenFinished`

this is to match that from the css side for users: https://developer.mozilla.org/en-US/docs/Web/CSS/animation-fill-mode