add jiff-datetime feature

[fdncred]

This is my first attempt at switching from the chrono crate to the jiff crate. It's behind a feature named jiff-datetime so that I can experiment with it in nushell.

[kronberger-droid]

Hey @fdncred,
whats the state of this?
I am just going around and checking on all the stale PRs.
I am totally fine with keeping this in the state it is if you want.
Just checking in.

[fdncred]

I'd like to migrate to jiff at some point here and in nushell and away from chrono but it doesn't have to be this PR.

[kronberger-droid]

Okay, then lets keep it open for discussion, or until it is superseded by another one.