input: Write a python function to implement a stack with push, pop, peek, is_empty, and size_stack methods.
  assert stack.is_empty() == True
  stack.push(5)
  assert stack.is_empty() == False
  assert stack.peek() == 5
  assert stack.size_stack() == 1
  assert stack.pop() == 5
  assert stack.size_stack() == 0
output: class Stack:
      def __init__(self):
          self.items = []
  
      def push(self, item):
          self.items.append(item)
  
      def pop(self):
          if not self.is_empty():
              return self.items.pop()
          raise IndexError("pop from empty stack")
  
      def peek(self):
          if not self.is_empty():
              return self.items[-1]
          raise IndexError("peek from empty stack")
  
      def is_empty(self):
          return len(self.items) == 0
  
      def size_stack(self):
          return len(self.items)
  
  # Testing the implementation
  stack = Stack()
  assert stack.is_empty() == True
  stack.push(5)
  assert stack.is_empty() == False
  assert stack.peek() == 5
  assert stack.size_stack() == 1
  assert stack.pop() == 5
  assert stack.size_stack() == 0