Add calendar consts for calendar construction

[nekevss]

This is primarily meant to cleanup the general API.

Previously, when using temporal_rs in Rust for non ISO calendars, you would have to use something like the below.

let date = PlainDate::try_new(2025, 5, 6, Calendar::new(AnyCalendarKind::Japanese));

Now, this is hopefully a bit more streamlined.

let date = PlainDate::try_new(2025, 5, 6, Calendar::JAPANESE)

Alternatively, we could take either AnyCalendarKind directly or a temporal_rs specific enum. My one hesitation about AnyCalendarKind is that it feels like an implementation detail / potential power user API leaking out into a general usage API. Definitely open to suggestions or other thoughts on the matter.