Move and reimplement `convert_number_to_words.py` (#8998)

* Move and reimplement convert_number_to_words.py

- Move convert_number_to_words.py from web_programming/ to conversions/
- Reimplement the algorithm from scratch because the logic was very
  opaque and too heavily nested
- Add support for the Western numbering system (both short and long)
  because the original implementation only supported the Indian
  numbering system
- Add extensive doctests and error handling

* updating DIRECTORY.md

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