Add support for relative durations in `exclude-newer` (#16814)
Adds support for "friendly" durations like, 1 week, 7 days, 24 hours
using Jiff's parser. During resolution, we calculate this relative to
the current time and resolve it into a concrete timestamp for the
lockfile. If the span has not changed, e.g., to another relative value,
then locking again will not change the lockfile. The locked timestamp
will only be updated when the lockfile is invalidated, e.g., with
`--upgrade`. This prevents the lockfile from repeatedly churning when a
relative value is used.
