Add TZ="timezone" support

[bezmuth]

This fixes: uutils/coreutils#7520
This is my first pr to the uutils repos so there is a good chance I've made a few mistakes, I'm not entirely sure if I've chosen the right name for functions/variables either. I can write some tests for the TZ= functionality but I want to make sure I haven't missed anything yet.

[cakebaker]

Can you please run cargo fmt and cargo clippy? Thanks! And yes, please write some tests :)

[cakebaker]

@yuankunzhang I have seen you started with the migration from chrono to jiff whereas this PR adds more code that uses chrono :| Is it ok if I merge this PR or would it cause too much hassle for what you are working on?

[yuankunzhang]

It is better for this PR to wait until the jiff migration is completed.

[yuankunzhang]

Hi @bezmuth , now that we have completed the chrono to jiff migration, could you please refactor your code based on the latest main branch? Thanks.

[bezmuth]

Yep, I'll get on it. Jiff seems a lot more robust for timezones

[cakebaker]

Closing this PR as the functionality has been added in #232

@bezmuth thanks anyway for your PR!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (25f9e48) to head (50bf621).
⚠️ Report is 50 commits behind head on main.

Additional details and impacted files

@@     Coverage Diff     @@
##   main   #200   +/-   ##
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