Expose wrapPromise and Thenable in the GraalJS public API

[mikehearn]

In the unit tests for promises we find:

private static Value wrapPromise(Context context, CompletableFuture<String> javaFuture) {
        Value global = context.getBindings("js");
        Value promiseConstructor = global.getMember("Promise");
        return promiseConstructor.newInstance((ProxyExecutable) arguments -> {
            Value resolve = arguments[0];
            Value reject = arguments[1];
            javaFuture.whenComplete((result, ex) -> {
                if (result != null) {
                    resolve.execute(result);
                } else {
                    reject.execute(ex);
                }
            });
            // return value of function(resolve,reject) is ignored by `new Promise()`.
            return null;
        });
    }

    public interface Thenable {
        void then(Value onResolve, Value onReject);
    }

These look useful! It'd be great if these conversions were done automatically, rather than being a bit of utility code.