Initial implementation for JsPromise and TaskBuilder

[kjvalencik]

Implements the JsPromise and TaskBuilder RFC.

Adds two new feature flags:

• promise-api
• task-api

Rust Docs

pub fn sum(mut cx: FunctionContext) -> JsResult<JsPromise> {
    let nums = cx.argument::<JsTypedArray<f64>>(0)?.as_slice(&cx).to_vec();
    let promise = cx
        .task(move || nums.into_iter().sum())
        .promise(|cx, n: f64| Ok(cx.number(n)));

    Ok(promise)
}

[dherman]

It's remarkable how clean the implementation is -- I bet the hardest part was figuring out the API, and then the primitives match the Node-API design enough that the implementation is pretty clean!

I made a couple tiny suggestions in async_work but I'll leave it to you.