[scroll-animations] Update scroll timeline css syntax and properties (w3c#4338, w3c#4336) (w3c#4751)

## Introduce @timeline at-rule

Allow animation-name property to be used to select timeline
Update examples to use new syntax

## Use shorted names for properties:
 startScrollOffset => start
 endScrollOffset   => end
 scrollSource      => source

## Minor changes to avoid bikeshed WARNING:
 - disambiguate reference to rAF
 - Reference layout cycle

Author: majido