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/Promise.cpp

@@ -116,7 +116,7 @@ Promise::ResolvingFunctions Promise::create_resolving_functions()
             dbgln_if(PROMISE_DEBUG, "[Promise @ {} / PromiseResolvingFunction]: Exception while getting 'then' property, rejecting with error", &promise);
             vm.clear_exception();
             vm.stop_unwind();
-            return promise.reject(then.throw_completion().value());
+            return promise.reject(*then.throw_completion().value());
         }
 
         // 11. Let thenAction be then.[[Value]].