Add .join() to JsContext and JsWorker

[alshdavid]

Add a .join() function to JsContext and JsWorker to enable explicit waiting on completion

let w0 = //

let ctx = w0.create_context();

// Use context

// Block async until context completes
ctx.join().await;

// Block until context completes
ctx.join_blocking();

// Block async until context completes
w0.join().await;

// Block until context completes
w0.join_blocking();