input: Write a python function to search for a substring in a given string (case insensitive) and return the starting index if found, otherwise return -1.
  assert search_substring("Welcome To Programming", "to")==8
  assert search_substring("Hello World", "world")==6
  assert search_substring("Python Programming", "java")==-1
output: def search_substring(main_string, sub_string):
      main_string_lower = main_string.lower()
      sub_string_lower = sub_string.lower()
      return main_string_lower.find(sub_string_lower)