Improve CLI robustness, depth handling, and ignore pattern supportImprove rptree cli

[davi-rf]

This PR improves the robustness, usability, and correctness of the CLI and tree generation logic.

Improvements

• Fixed depth-level behavior to correctly reflect the intended tree depth
• Enhanced --ignore option to support pattern matching using fnmatch
• Added warning when .gitignore support is requested but pathspec is not installed
• Replaced exit() with sys.exit() for proper script termination
• Improved overall code quality and consistency

Notes

• Existing behavior is preserved
• No breaking changes introduced

These changes improve usability while keeping the tool simple and aligned with its original purpose.
I have already tested it and it's working well.
Ready to merge.