Basic support for diary sexp entries

[przemekd]

Summary

This PR adds basic support for diary sexp entries

Closes #195

Changes

• List changes here

Checklist

I confirm that I have:

• [ ] Followed the Conventional Commits specification (e.g., feat: add new feature, fix: correct bug, docs: update documentation).
• [ ] My PR title also follows the conventional commits specification.
• [ ] Updated relevant documentation, if necessary.
• [ ] Thoroughly tested my changes.
• [ ] Added tests (if applicable) and verified existing tests pass with make test.
• [ ] Checked for breaking changes and documented them, if any.