[css-animations-2] Accept 'auto' for animation-duration in shorthand

[cdoublev]

2a7cc4b added auto to animation-duration but <single-animation> only accept <time>.

Name: animation-duration
Value: [ auto | <<time [0s,∞]>> ]#

<single-animation> = <time [0s,∞]> || <easing-function> || <time> || <single-animation-iteration-count> || <single-animation-direction> || <single-animation-fill-mode> || <single-animation-play-state> || [ none | <keyframes-name> ]

Note that order is important within each animation definition: the first value in each <single-animation> that can be parsed as a <time> is assigned to the animation-duration

https://drafts.csswg.org/css-animations-1/#typedef-single-animation

[fantasai]

Hi @cdoublev ! Because of its parsing complications, I think any changes or additions to the animation shorthand need to go through WG discussion, so maybe we need to open an issue first?

[cdoublev]

Superseded by 5feda7d.