Add TryFrom for PartialDuration to Duration

[nekevss]

This PR adds a TryFrom implemenation for converting a PartialDuration to Duration.

The core reasoning is to simplify the case where someone would want to add a single day in Rust.

use temporal_rs::{Temporal, PlainDate, partial::PartialDuration};

let today = Temporal::now().plain_date_iso(None).unwrap();
let one_day = PartialDuration::empty().with_days(1);
// Before: 
// let tomorrow = today.add(Duration::from_partial_duration(one_day).unwrap()).unwrap()
// After:
let tomorrow = today.add(one_day.try_into().unwrap()).unwrap();