date: support relative `--date`

[PedroHLC]

A simple example of inputs/outputs of what GNU's Coreutils is capable of is visible at https://www.cyberciti.biz/tips/linux-unix-get-yesterdays-tomorrows-date.html

This issue was suggested by @tertsdiepraam in #2685 (comment) (which might be interesting to solve in the same PR).

[Ecordonnier]

I think this was solved by #4790

[cakebaker]

Yes, #4790 implements most of the functionality, though there is still some functionality missing (for example, "next week" and "last week" are not recognized yet).

[jfinkels]

There is now some support for specifying a relative time. For example:

$ TZ=UTC0 ./target/debug/date
Fri Feb 21 23:04:05 UTC 2025
$ TZ=UTC0 ./target/debug/date --date=tomorrow
Sat Feb 22 23:04:10 UTC 2025

The code is in the parse_datetime library.

I suggest we close this issue as "done enough" and open new ones for specific bugs.