Add a quicklook plugin for macOS.

[jtkiley]

Quicklook on macOS is really helpful with notebooks, and the one small app that includes a plugin Jupyter Notebook Viewer is not notarized (and not clear whether that will ever happen).

With VS Code taking over the file association on macOS for .ipynb files (which is great; much better than explaining to new users that double-clicking doesn't work), this seems like a next logical step to making Jupyter nicer to work with on macOS for a lot of users.

It appears that there's a js library for this already, and the quicklook plugin used by that app is MIT licensed, so it may be pretty easy for someone who knows how those parts fit together.