docx writer: fully support for \mathop{}

I want to write `\varlimsup_{n\to\infty}` in markdown and convert it to word

$$\varlimsup_{n\to\infty}$$

but I got an error when I convert, I think the reason is pandoc don't know how to convert it. so I use `\mathop{\overline{\lim}}`, but the result is wrong

$$\mathop{\overline{\lim}}_{n\to\infty}$$

and I try `\underset{n\to\infty}{\overline{\lim}}`, it works

so I hope pandoc can support `\mathop{}` correctly

this image is an example of what I say above: