fix: capitalization logic

[Chinmay-KB]

This pull request focuses on improving the string capitalization logic in the stringr package, making the capitalize method more robust and consistent, especially for strings that are already in uppercase or have mixed casing. It also updates the version and adds relevant test cases to ensure the new behavior works as intended. Fixes #9

Enhancements to string capitalization:

• Improved the capitalize method in case.dart so that if the first character is already uppercase, the rest of the string is converted to lowercase, ensuring consistent capitalization for strings like "HELLO" or "HeLLo".
• Updated the logic in the case extension to handle words that are fully uppercase, preserving them as-is when appropriate.

Testing improvements:

• Added new test cases in case_test.dart to verify the updated capitalize behavior for all-uppercase and mixed-case strings.

Version update:

• Bumped the package version from 1.2.1 to 1.2.2 in pubspec.yaml to reflect these changes.