Enable `Pre(...)` on full stataments

Currently, when creating events where teh affect depends on multiple symbolic variables, you have to do `Pre` on each, e.g.
```julia
@variables X(t)
@parameters Xup
event = [1.0] => [X ~ Pre(X) + Pre(Xup)]
```
if would be much easier if you could just `Pre` the entire RHS:
```julia
event = [1.0] => [X ~ Pre(X + Xup)]
```