[css-values-5] Syntax of progress() vs mix() functions

We currently have the `progress()` functions defined like

```
progress(<value> from <start> to <end>)
```

but the `mix()` functions are defined like

```
mix(<progress>, <start>, <end>)
```

Would it make sense to align these a bit more, e.g.

```
mix(<progress> of <start> to <end>)
```
?