Add last_digit function with type hints and doctests

[prachi757]

Describe your change:

• Add a new algorithm

This pull request adds a function last_digit in the maths/special_numbers directory.

Key highlights:

• Returns the last digit of any integer (supports both positive and negative values)
• Includes proper Python type hints
• Comes with clear and passing doctests
• Fully follows TheAlgorithms Python contribution guidelines and style rules

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Checklist:

• I have read CONTRIBUTING.md.
• This pull request is entirely my own work — no part is plagiarized.
• This PR only affects one algorithm file.
• File is inside an existing directory and properly named in lowercase.
• Function and variable names follow Python conventions.
• All parameters and return values are annotated with type hints.
• Function includes doctests that pass automated testing.

[algorithms-keeper]

Click here to look at the relevant links ⬇️

🔗 Relevant Links

Repository:

• Contributing guidelines
• Project Euler solution guidelines

Python:

• Formatted string literals (f-strings)
• Type hints
• doctest
• unittest
• pytest

Automated review generated by algorithms-keeper. If there's any problem regarding this review, please open an issue about it.

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[algorithms-keeper]

Please provide descriptive name for the parameter: n