fix(ui): allow built-in completions in orgmode.ui.input

[troiganto]

Summary

Allow ui.input to take not only functions for custom completion, but also strings for buil-in completion.

The org-attach feature uses both the dir built-in completion (to choose a directory to attach) and the buffer built-in completion (to choose a buffer whose contents are to be attached).

Related Issues

Extracted from #878

Changes

• adjust type signature of orgmode.ui.input.open() to allow string-type completion arguments
• special-case on type(completion) being a string.

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]

Last unrelated PR to be extracted from the org-attach feature, I think :slight_smile:

My next step will be to see if I can extract an MVP from the org-attach branch that is easier to review