Add slicing exercises for lists and tuples in ch04 #95

The current ch04 exercises cover string slicing well, but there are
no dedicated exercises for slicing lists and tuples.

It would be helpful to add exercises covering:

• Basic list slicing [start:stop:step]
• Negative indexing on lists
• Tuple slicing
• Reversing a list using slicing [::-1]

This would give beginners a more complete understanding of slicing
across all sequence types in Python.