Add ascii_to_char.py to conversions module

[Alessio-Falcini]

Closes #13569

This PR adds a new script ascii_to_char.py to the conversions module with:

• ascii_to_char(): converts ASCII integer to character
• string_to_ascii(): converts string to list of ASCII values
• ascii_list_to_string(): converts ASCII list back to string

All functions include comprehensive docstrings, error handling, and doctests (25 tests passing).