[css-anchor-position] anchor() arguments should be reorderable

The syntax of `anchor()` is defined as follows:

```
<anchor()> = anchor( <anchor-element>? <anchor-side>, <length-percentage>? )
```

However we have a CSS design principle that if syntax can be unambiguously reordered, it should be allowed to be reordered. Shouldn't this be

```
<anchor()> = anchor( [ <anchor-element>? && <anchor-side> ], <length-percentage>? )
```
?