Test wasm-bindgen feature on CI and fix implementation

[pitdicker]

This is probably going to take a couple of tries...

[pitdicker]

Tests are running! But the methods in offset::local look quite wrong...

[pitdicker]

offset_from_local_datetime for wasm_bindgen is now as correct as I can make it.

[pitdicker]

I have tested it locally with:

#[wasm_bindgen_test]
fn amsterdam_datetime() {
    let dt = NaiveDate::from_ymd_opt(2023, 06, 06).unwrap().and_hms_opt(16, 0, 0).unwrap();
    let res = Local.from_local_datetime(&dt).single().unwrap();
    assert_eq!(res.offset().fix(), FixedOffset::east_opt(2 * 60 * 60).unwrap());

    let dt = NaiveDate::from_ymd_opt(2023, 01, 01).unwrap().and_hms_opt(16, 0, 0).unwrap();
    let res = Local.from_local_datetime(&dt).single().unwrap();
    assert_eq!(res.offset().fix(), FixedOffset::east_opt(1 * 60 * 60).unwrap());

    let dt = NaiveDate::from_ymd_opt(-300, 01, 01).unwrap().and_hms_opt(16, 0, 0).unwrap();
    let res = Local.from_local_datetime(&dt).single().unwrap();
    assert_eq!(res.offset().fix(), FixedOffset::east_opt(17 * 60).unwrap());

    // Should return `LocalResult::None`, but we get a result anyway.
    let dt = NaiveDate::from_ymd_opt(2023, 03, 26).unwrap().and_hms_opt(0, 2, 30).unwrap();
    let res = Local.from_local_datetime(&dt).single().unwrap();
    assert_eq!(res.offset().fix(), FixedOffset::east_opt(1 * 60 * 60).unwrap());
}

But I don't think we can make this part of the test or CI as the local timezone may be different.

[djc]

When/how did this become unnecessary?

[pitdicker]

The platform-specific code still lives in Utc::now. This could have been part of #1072.
At the time of #331 Local::now depended on time::now.

[djc]

Why do we need to set TZ and NOW here? Maybe add a comment?

[pitdicker]

Added a comment