feat(files): add method OrgHeadline:toggle_tag()

[troiganto]

Summary

This PR adds a method OrgHeadline:toggle_tag().

Right now, this simply moves the code of OrgMappings.toggle_archive_tag() into the OrgHeadline class. However, the org-attach module has a feature org_attach_auto_tag, that enables, disables and toggles the tag :ATTACH: when managing attachments. It's used all over the place. (1, 2, 3, 4, 5, 6, 7)

Related Issues

Extracted from #878

Changes

• add method OrgHeadline:toggle_tag(tag: string, onoff?: boolean)
• use it in OrgMappings:toggle_archive_tag()
• add tests in headline_spec.lua

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.

[troiganto]

(Unrelated, but thanks for keeping up with this rapid pace of PRs! As a chronic procrastinator, I can imagine it's not easy 👏)

[kristijanhusak]

Thanks!