LibJS: Use a Variant instead of two Optionals for ThrowCompletionOr

Comes with the usual benefit of saving some space on the stack, as well
as making a situation where both or neither Optionals hold a value
impossible.

The various unwrapping additions are required as we can no longer
construct a ThrowCompletionOr<T> from an Optional<T> - rightfully so.

Userland/Libraries/LibWeb/HTML/CrossOrigin/AbstractOperations.cpp

@@ -186,7 +186,7 @@ JS::ThrowCompletionOr<JS::Value> cross_origin_get(JS::VM& vm, JS::Object const&
 
     // 3. If IsDataDescriptor(desc) is true, then return desc.[[Value]].
     if (descriptor->is_data_descriptor())
-        return descriptor->value;
+        return *descriptor->value;
 
     // 4. Assert: IsAccessorDescriptor(desc) is true.
     VERIFY(descriptor->is_accessor_descriptor());