Re: [csswg-drafts] [css-ui] Add a property controlling the navigation order

For defining the visual order, the value could be like: 

```css
nav-order: strutural | row | row-reverse | column | column-reverse
```
`row`, `row-reverse`, `column`, `column-reverse` are also used in [flex-direction](https://www.w3.org/TR/css-flexbox-1/#propdef-flex-direction).
Those can be combined well with the writing mode, as defined [here](https://www.w3.org/TR/css-flexbox-1/#axis-mapping).

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