public static Future<String> newerVersionAsync() {
		if (shouldCheck()) {
			return retrieveLatestVersionAsync();
		}
		return null;
	}
Asynchronously retrieve the latest JBang version number but only if enough
time has passed since the last time we checked. The result of this function
can be used together with `showMessage()` to inform the user.

@return A future to obtain the latest version number or `null` if not enough
        time has passed since the last check.
src/main/java/dev/jbang/util/VersionChecker.java