Write unlock() function

[adigulalkari]

Write a function to use the old book to unlock the treasure box.
The treasure box can be unlocked only if the user is in the bedroom and has "old_book" in the inventory
Display the "puzzle" any time the user tryies to unlock the treasure box.
Return "You read the old book and found a code. The locked treasure box has been unlocked." once unlocked
Increase 50 points if the tresure box gets unlocked.
Return appropriate message if any of the conditions are not met.