DRAFT: "async_local" feature for local-thread Rust futures

[alshdavid]

This PR adds the ability to use the local thread to manage concurrency allowing for faster and more ergonomic syntax

Changelog

• async_local feature
• adds env.spawn_local
• adds JsRc

Usage:

#[napi]
pub fn run_callback(env: Env, callback: JsFunction,
) -> napi::Result<JsUndefined> {
  let callback = callback.into_ref()?;

  env.spawn_local(move |env| async move {
    let callback = callback.clone(&env).unwrap();
    callback.call_without_args(None).unwrap();
  });

  env.get_undefined()
}

For more details see Github RFC here

[alshdavid]

Moved to here
#2158