Request: add mv in library_fs.js

It would be nice to be able to do a mv operation in library_fs.js, specially one that can move directories even if they have files and other directories inside of it.

https://github.com/emscripten-core/emscripten/issues/4952#issuecomment-1435327297

The need arise when I was trying to figure a sensible approach for deleting the permanent files in IDBFS, and it felt like I don't want to really delete right away, but first move to a recycle bin like location to evaluate if the deletion had the expected result and later actually delete them from the recycle bin. But it turns out I can't figure out how to move the files/directories.