input: Write a python function to convert a given string to ASCII art based on a predefined dictionary.
  assert ascii_art("Hello") == '   //\\/ \\\n\\/3_  3_\\\n  (__/ \\__)\n\\/2_  2_\\\n  (__/ \\__)\n'
  assert ascii_art("World") == '  (___/ \\___)\n \/3_  3_\\/\n  (__/ \\__)\n\\/2_  2_\\\n \\__/ .- \\_/\n'
output: def ascii_art(text):
      art_dict = {
          'H': '   //\\/ \\',
          'e': '\\/3_  3_\\',
          'l': '  (__/ \\__)',
          'o': '\\/2_  2_\\',
          'W': '  (___/ \\___)',
          'r': '  (__/ \\__)',
          'd': ' \\__/ .- \\_/'
      }
  
      result = '\n'.join(art_dict[char] for char in text if char in art_dict)
      return result