Add hard line wrap mode for Markdown and Raw cells

[joelostblom]

Problem

Currently all the line wrapping options in JupyterLab appear to only support soft line wrapping and not actually change the source document by inserting newlines (hard line wrap). Hard line wraps makes it easier to review code locally and on GitHub since there is no endless horizontal scrolling to read an entire paragraph all on a single line. Compare the following example:

# Soft/No wrap
JupyterLab can be extended using npm packages that use our public APIs. The prebuilt extensions can be distributed via PyPI, conda, and other package managers. The source extensions can be installed directly from npm (search for jupyterlab-extension) but require additional build step. You can also find JupyterLab extensions exploring GitHub topic jupyterlab-extension. To learn more about extensions, see the user documentation.

# Hard wrap
JupyterLab can be extended using npm packages that use our public APIs. The
prebuilt extensions can be distributed via PyPI, conda, and other package
managers. The source extensions can be installed directly from npm (search for
jupyterlab-extension) but require additional build step. You can also find
JupyterLab extensions exploring GitHub topic jupyterlab-extension. To learn
more about extensions, see the user documentation.

Proposed Solution

I suggest adding a checkbox in the setting editor that enables hard wrap instead of soft wrap. This would probably only make sense for the Markdown and Raw Cell Configuration, I can't imagine a scenario where one would want hard wrap for code since that would change the functionality.

The checkbox text could be "Use hard instead of soft line wrap" and it could be added just under the "Line Wrap" dropdown.