feat(neon): Add neon::types::extract::Boxed for JsBox conversions

[kjvalencik]

JsBox<T> can be extract in an exported function.

#[neon::export]
fn example(boxed: Handle<JsBox<String>>) {}

However, this only works for synchronous functions. If you need boxed data in an async function or task, it requires first cloning it in a sync function, which can be verbose.

#[neon::export]
fn example<'cx>(cx: &mut FunctionContext<'cx>, s: Handle<'cx, JsBox<String>>) -> Handle<'cx, JsPromise> {
    let s = String::clone(&s);
    
    cx.task(move || s).promise(|mut cx, s| cx.string(s))
}

However, we can trivially implement TryFromJs where T: Clone. It must be clone because JsBox only allows immutable references. This allows a much more ergonomic version.

#[neon::export(task)]
fn example(Boxed(s): Boxed<String>) -> String {
    s
}

Similarly, we can implement TryIntoJs for Boxed by calling cx.boxed(..). The TryIntoJs implementation does not require Clone because we already have an owned copy.

[dherman]

This is very nice, and the name is perfect, given the symmetry with cx.boxed().