Implement `ver_before()` and `ver_after()`

These functions would take a version code and a vector of those and return the previous or the next version code than the target one. Like that:

```r
ver_before("1.4.0", c("1.3.1", "1.5.0", "1.4.0", "1.3.2"))
# [1] "1.3.2"
```

In general, this would work like applying `ver_latest()` on versions that are `ver_is_older()` (and analogously for `ver_after()`).