feat: NoteAdded event

[celsobenedetti]

Summary

Small PR adding new NoteAdded event.

Related Issues

Closes #1102

Changes

• Add NoteAdded event.
• Expose them in require('orgmode.events').events
• Hook the event into Headline:add_note
• Add tests

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.

[kristijanhusak]

Thanks!