Add ReflectionFunctionAbstract::isAnonymous()

Hi all, Given the addition of Closure::fromCallable() and the upcoming first-class callable syntax in 8.1, it seems slightly problematic that there's no simple way to tell by reflection if a Closure refers to an anonymous function or not. ReflectionFunctionAbstract::isClosure() (perhaps somewhat misleadingly) returns whether the closure is literally a \Closure instance, so it's not useful for this purpose. The only way to do this currently (that I know about) is to check if the name of the function contains "{closure}", which is a bit unpleasant and depends on undocumented behaviour. I'm proposing the addition of ReflectionFunctionAbstract::isAnonymous(), which would fill this use case, and may be able to offer an implementation. Thanks, Dylan Taylor.