LibJS: Return Optional<T> from Completion::{value,target}(), not T

In the end this is a nicer API than having separate has_{value,target}()
and having to check those first, and then making another Optional from
the unwrapped value:

    completion.has_value() ? completion.value() : Optional<Value> {}
    //                       ^^^^^^^^^^^^^^^^^^
    //         Implicit creation of non-empty Optional<Value>

This way we need to unwrap the optional ourselves, but can easily pass
it to something else as well.

This is in anticipation of the AST using completions :^)

Userland/Libraries/LibJS/Runtime/AsyncFunctionDriverWrapper.cpp

@@ -41,7 +41,7 @@ ThrowCompletionOr<Value> AsyncFunctionDriverWrapper::react_to_async_task_complet
         vm.clear_exception();
         vm.stop_unwind();
         auto promise = Promise::create(global_object);
-        promise->reject(generator_result.throw_completion().value());
+        promise->reject(*generator_result.throw_completion().value());
         return promise;
     }