[css-color-5] Add the none keyword to light-dark()

#12513 (comment)

This means that `light-dark(url(foo.png), none)` will be a valid value.
The `none` keyword is equivalent to `linear-gradient(transparent)`, but is more intuitive.

This also makes conditional checks in `@supports` more concise:

```css
@supports (light-dark(none, none)) { }
```

In addition, `none` is also supported in `-webkit-cross-fade()`, See:
https://github.com/web-platform-tests/wpt/blob/0efed9003fb6c920d404928776fc9a400bb78560/css/css-images/cross-fade-legacy-crash.html#L8

Author: yisibl

css-color-5/Overview.bs

@@ -2795,7 +2795,7 @@ or any other color or monochrome output device which has been characterized.
 	<pre class='prod'>
 		<dfn export>light-dark()</dfn> = <<light-dark-color>> | <<light-dark-image>>
 		<dfn export><<light-dark-color>></dfn> = light-dark(<<color>>, <<color>>)
-		<dfn export><<light-dark-image>></dfn> = light-dark(<<image>>, <<image>>) 
+		<dfn export><<light-dark-image>></dfn> = light-dark(<<image>> | none, <<image>> | none) 
 	</pre>
 
 	For the color form, this function computes to the computed value of the first color,