feat(utils): add optional flag to utils.writefile() for exclusive writes

[troiganto]

Summary

This PR adds an optional flag to utils.writefile() to enable exclusive writes. Such a write fails if a file with the given name already exists.

The org-attach feature requires functionality like this because it is really adamant about not overwriting attachments that the user might not be aware of. Example here.

Related Issues

Extracted from #878

Slowly getting there …

Changes

• Add new optional parameter opts to utils.writefile(). Right now, the only option is a boolean flag excl.
• Add tests for utils.writefile()
• Add tests for utils.readfile()
• Also add a tiny test for folds; I was debugging an issue with folding earlier (turns out lazy loading was the culprit) and since there are no tests of folding at all rn, I didn't want to throw it away 🙂

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!