Spy Message Decoder

A spy hides a code word inside an ordinary sentence. A second list says which positions to read, in order.

Task: write decode(message, positions) that returns a new string made from the letter at each position in positions, in the order given.

• Positions can be negative, which counts from the end, just like a list.
• A position can appear more than once.
• Every position is valid for the message. Spaces count as positions too.
• If positions is empty, return "".