new crossrefs+flextable+docx interaction

[cscheid]

In .docx format, crossreferenceable code cells with flextable outputs cause Microsoft Word to emit a warning about a malformed document.

This doesn't happen when the code cell isn't crossreferenceable, so we're doing something bad to it.

[cscheid]

This is part of #4944.

[cscheid]

I spent a couple of hours trying to figure out what is going wrong and failed here - I can't tell what we might be emitting that's bad.

[cscheid]

Let's check non-table crossrefs (duh)