Add phone number validator using regex

[ShashankChilukuri]

This pull request adds a new algorithm to validate Indian 10-digit phone numbers using regular expressions.

✅ The function uses Python's re.fullmatch() to match valid phone numbers.

✅ It checks that the phone number starts with 6, 7, 8, or 9 and contains exactly 10 digits.

✅ The function is defined as validate_phone_number(phone: str) -> bool.

✅ It includes multiple doctests for automatic validation.

✅ The file is located in strings/phone_number_validator.py and follows the project’s coding and contribution standards.

This function is useful for basic input validation in form processors, mobile apps, and backend systems that deal with Indian users.