Join method

[alshdavid]

pub fn main() -> anyhow::Result<()> {
    let runtime = JsRuntime::initialize_once(JsRuntimeOptions::default())?;
    let worker = runtime.spawn_worker(JsWorkerOptions::default())?;
    let context = worker.create_context()?;

    context .eval("'Some JavaScript Code'")?;

    // New
    context .join()?;
    worker .join()?;

    Ok(())
}