Function spawn 

pub fn spawn<F, T>(f: F) -> JoinHandle<T>
where
    F: FnOnce() -> T + Send + 'static,
    T: Send + 'static,

Spawn a new thread.

On WASM, uses [wasm_safe_thread::Builder] with an explicit shim_name so workers can locate the wasm-bindgen JS shim for initSync.