[semidet]is_asyncTrue when we can call await/2
in the current state. This implies Prolog has been called from
JavaScript code that is prepared to deal with Prolog yielding and Prolog
is not inside a callback from C (WASM).
[semidet]is_asyncTrue when we can call await/2.
We can not yield when we are in a callback from C (WASM)
to Prolog.