input: Write a python function to validate usernames based on given criteria.
  assert is_valid_username("abcdefgh")
  assert not is_valid_username("1abcdefgh")
  assert is_valid_username("user_name_123")
  assert not is_valid_username("short")
  assert not is_valid_username("invalid-username")
output: def is_valid_username(username):
      import re
      
      # Criteria: 
      # 1. Must be between 8 and 30 characters long
      # 2. Must start with a letter
      # 3. Can contain letters, numbers, and underscores
      pattern = r'^[a-zA-Z][a-zA-Z0-9_]{7,29}$'
      
      return bool(re.match(pattern, username))