Depecrate "skip all"

The feature makes little sense, allowing skipping of "all supported traits", a group that cannot be expanded in a non-breaking change.

This will still keep the possibility to skip all traits directly, just remove the "skip all" behavior. I.e. use 
```rs
#[derive_where(skip(Debug, EqHashOrd, Zeroize /*, Clone */))]
```
instead of
```rs
#[derive_where(skip)]
```