feat(napi): add threadsafe deferred values

[devongovett]

Depends on #1303. See last commit for actual diff.

Closes #1304.

This adds an env.create_deferred method, which returns a new JsDeferred value and a JS Promise. The deferred can be used to resolve or reject the promise from a background thread, using a thread safe function. A function is passed to the resolve method of a deferred, which is called from the JS thread to create a JS value. This can return any type conforming to the ToNapiValue trait.

Example:

#[js_function(0)]
pub fn example(ctx: CallContext) -> Result<JsObject> {
  let (deferred, promise) = ctx.env.create_deferred()?;

  std::thread::spawn(move || {
    // ...
    deferred.resolve(|env| Ok(15));
  });

  Ok(promise)
}

I have also updated execute_tokio_future to utilize this new primitive instead of the existing internal promise module, which worked similarly but was more limited.