Describe your change:

• Add an algorithm

This PR adds a new function last_digit under the maths/special_numbers directory.

Details:

• Returns the last digit of a given integer (positive or negative)
• Uses proper Python type hints
• Includes well-written doctests that pass all automated tests
• Complies with naming and formatting conventions

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, I have not combined unrelated changes.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• This algorithm does not include an external reference URL, as it's a basic concept.