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

[ErdaradunGaztea]

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:

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()).