Added missing animation-fill-mode property

Added missing [`animation-fill-mode`](https://developer.mozilla.org/en-US/docs/Web/CSS/animation-fill-mode) property to `propertyKeywords_` array. Oddly, it was the only CSS animation property missing.

I also resorted the next few lines so one line would not be longer the all the others. :wink:

All tests passed, and `bin/lint` did not complain about anything.