CSS: Support comma-separated values for `animation*` properties

[AtkinsSJ]

Elements can have multiple animations at once, so the animation properties accept comma-separated lists of values. Currently we only accept a single value for each of these:

• animation
• animation-name
• animation-duration
• animation-timing-function
• animation-iteration-count
• animation-direction
• animation-play-state
• animation-delay
• animation-fill-mode

This works similarly to background and its longhand properties.

Each animation listed by name should have a corresponding value for the other animation properties listed below. If the lists of values for the other animation properties do not have the same length, the length of the animation-name list determines the number of items in each list examined when starting animations. The lists are matched up from the first value: excess values at the end are not used. If one of the other properties doesn’t have enough comma-separated values to match the number of values of animation-name, the UA must calculate its used value by repeating the list of values until there are enough. This truncation or repetition does not affect the computed value.

• https://www.w3.org/TR/css-animations-1/#animation-name

As part of this, we'll also need to be able to start multiple animations at once for an element, instead of just one.