[9.x] Arr::forget() for nested ArrayAccess objects

[sjokkateer]

Currently, calling Collection::pull() using a dot notatation key when dealing with nested ArrayAccess::class objects would result in the correct value being returned but it would not remove the key/value from the objects afterwards.
This is because the original call is forwarded to Arr::pull(), which calls Arr::forget(), and in Arr::forget() line 288 the conditional tests only for the value being an array through is_array.
Changing this part of the conditional to Arr::accessible() makes the method return not only the correct value but also remove the corresponding key when dealing with nested ArrayAccess::class objects.

If I am not mistaken this change should be fully backward compatible.

If there is anything I overlooked please let me know!