[css-display] Consider visual-order as an alias of order

[fantasai]

There's a lot of confusion over what order does, to the point that we drafted up a visual-order longhand alongside reading-order, instead of creating a shorthand for order and reading-order (which would have been more natural if not for the naming).

Should we alias order to visual-order to let stylesheets be more clear about what they're doing (eventually)?