Word wrapping for print view in HTML output

[PetteriAimonen]

Currently nbconvert HTML output never word-wraps cells.
This causes clipped output when printing the resulting file to PDF or paper, if there are long code lines or comments.

I would suggest something like this in default CSS:

pre {
  white-space: pre-wrap;
  word-wrap: break-word;
}

Optionally it could be made conditional on @media print.

(Originally reported in jupyterlab/jupyterlab#15500)